[ci] Build and pack updates, bump to .NET 5.0.201

[pjcollins]

I've been encountering various issues when building and packing .NET 6
content with dotnet build.

Using an older version of dotnet with <Exec/> tasks that invoke a
newer version of dotnet seems to be problematic:

(CreateAllPacks target) -> 
  /Users/builder/.dotnet/sdk/5.0.201/Sdks/Microsoft.NET.Sdk/targets/Microsoft.PackageDependencyResolution.targets(241,5): error MSB4018: The "ResolvePackageAssets" task failed unexpectedly. [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Runtime.proj] [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Sdk.proj]
  /Users/builder/.dotnet/sdk/5.0.201/Sdks/Microsoft.NET.Sdk/targets/Microsoft.PackageDependencyResolution.targets(241,5): error MSB4018: System.IO.FileNotFoundException: Could not load file or assembly 'NuGet.ProjectModel, Version=5.9.0.7122, Culture=neutral, PublicKeyToken=31bf3856ad364e35'. The system cannot find the file specified. [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Runtime.proj] [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Sdk.proj]
  /Users/builder/.dotnet/sdk/5.0.201/Sdks/Microsoft.NET.Sdk/targets/Microsoft.PackageDependencyResolution.targets(241,5): error MSB4018: [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Runtime.proj] [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Sdk.proj]
  /Users/builder/.dotnet/sdk/5.0.201/Sdks/Microsoft.NET.Sdk/targets/Microsoft.PackageDependencyResolution.targets(241,5): error MSB4018: File name: 'NuGet.ProjectModel, Version=5.9.0.7122, Culture=neutral, PublicKeyToken=31bf3856ad364e35' [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Runtime.proj] [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Sdk.proj]
  /Users/builder/.dotnet/sdk/5.0.201/Sdks/Microsoft.NET.Sdk/targets/Microsoft.PackageDependencyResolution.targets(241,5): error MSB4018: at Microsoft.NET.Build.Tasks.ResolvePackageAssets.CacheReader.CreateReaderFromMemory(ResolvePackageAssets task, Byte[] settingsHash) [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Runtime.proj] [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Sdk.proj]
  /Users/builder/.dotnet/sdk/5.0.201/Sdks/Microsoft.NET.Sdk/targets/Microsoft.PackageDependencyResolution.targets(241,5): error MSB4018: at Microsoft.NET.Build.Tasks.ResolvePackageAssets.CacheReader..ctor(ResolvePackageAssets task) [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Runtime.proj] [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Sdk.proj]
  /Users/builder/.dotnet/sdk/5.0.201/Sdks/Microsoft.NET.Sdk/targets/Microsoft.PackageDependencyResolution.targets(241,5): error MSB4018: at Microsoft.NET.Build.Tasks.ResolvePackageAssets.ReadItemGroups() [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Runtime.proj] [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Sdk.proj]
  /Users/builder/.dotnet/sdk/5.0.201/Sdks/Microsoft.NET.Sdk/targets/Microsoft.PackageDependencyResolution.targets(241,5): error MSB4018: at Microsoft.NET.Build.Tasks.ResolvePackageAssets.ExecuteCore() [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Runtime.proj] [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Sdk.proj]
  /Users/builder/.dotnet/sdk/5.0.201/Sdks/Microsoft.NET.Sdk/targets/Microsoft.PackageDependencyResolution.targets(241,5): error MSB4018: at Microsoft.NET.Build.Tasks.TaskBase.Execute() [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Runtime.proj] [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Sdk.proj]
  /Users/builder/.dotnet/sdk/5.0.201/Sdks/Microsoft.NET.Sdk/targets/Microsoft.PackageDependencyResolution.targets(241,5): error MSB4018: at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute() [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Runtime.proj] [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Sdk.proj]
  /Users/builder/.dotnet/sdk/5.0.201/Sdks/Microsoft.NET.Sdk/targets/Microsoft.PackageDependencyResolution.targets(241,5): error MSB4018: at Microsoft.Build.BackEnd.TaskBuilder.ExecuteInstantiatedTask(ITaskExecutionHost taskExecutionHost, TaskLoggingContext taskLoggingContext, TaskHost taskHost, ItemBucket bucket, TaskExecutionMode howToExecuteTask) [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Runtime.proj] [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Sdk.proj]
  /Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Directory.Build.targets(81,5): error MSB3073: The command "/Users/builder/Library/Android/dotnet/dotnet pack -p:Configuration=Release -p:NuGetLicense=/Users/builder/azdo/_work/2/s/xamarin-android/external/monodroid/tools/scripts/License.txt -p:AndroidRID=android-arm -p:AndroidABI=armeabi-v7a-net6 "/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Runtime.proj"" exited with code 1. [/Users/builder/azdo/_work/2/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Sdk.proj]

The same scenario fails even earlier if the nested project build also
targets a newer .NET SDK version:

BuildILLinkCustomStep:
  C:\Users\Peter\android-toolchain\dotnet\dotnet build -p:Configuration=Release "C:\Users\Peter\source\pj\xamarin-android\src\Microsoft.Android.Sdk.ILLink\Microsoft.Android.Sdk.ILLink.csproj"
  Microsoft (R) Build Engine version 16.10.0-preview-21175-01+afd0b6210 for .NET
  Copyright (C) Microsoft Corporation. All rights reserved.

  Determining projects to restore...
  C:\Program Files\dotnet\sdk\5.0.201\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.TargetFrameworkInference.targets(141,5): error NETSDK1045: The current .NET SDK does not support targeting .NET 6.0.  Either target .NET 5.0 or lower, or use a version of the .NET SDK that supports .NET 6.0. [C:\Users\Peter\source\pj\xamarin-android\src\Microsoft.Android.Sdk.ILLink\Microsoft.Android.Sdk.ILLink.csproj] [C:\Users\Peter\source\pj\xamarin-android\build-tools\create-packs\Microsoft.Android.Ref.proj]

We do not need to build any of our .NET 6 content with .NET 6. All of
our .NET 6 sources and dependencies can be built and packed with a
target framework of netstandard2.0 or net5.0. To fix these issues
and simplify things, our .NET 6 build and packaging targets have been
updated to use the system dotnet. This version has been bumped to
.NET 5.0.201 in CI.

A new make rule create-nupkgs has been added to reduce the number of
places where we were previously calling msbuild /t:CreateAllPacks in
yaml. The create-installers rule now depends on this new rule.

The Windows Dotnet Build and Smoke Test job has been updated to run
tests against the netcoreapp3.1 version of Xamarin.Android.Build.Tests.
A duplicate solution build has also been removed, as the PackDotNet
target will build the entire Xamarin.Android.sln.

A duplicate file reference for Java.Interop.Tools.Generator.dll has
been removed from the Microsoft.Android.Sdk packs.

[jonathanpeppers]

Should we put a note, we could potentially build this project normally when we get #5806? Then msbuild on Mac could build net5.0, and we could put it in Xamarin.Android.sln like a normal project?

[pjcollins]

I dropped a note on the other PR, we can potentially include these changes there 👍

[jonathanpeppers]

Otherwise LTGM 👍