Temporarily revert Microsoft.AspNetCore.HttpOverrides to netstandard2.0 (#4446)

We're running into issues getting IIS tests on netcoreapp3.0. This unblocks the build and gives us more time to work on #4371

Author: natemcmaster

src/Middleware/HttpOverrides/src/Microsoft.AspNetCore.HttpOverrides.csproj

@@ -4,7 +4,7 @@
     <Description>ASP.NET Core basic middleware for supporting HTTP method overrides. Includes:
 * X-Forwarded-* headers to forward headers from a proxy.
 * HTTP method override header.</Description>
-    <TargetFramework>netcoreapp3.0</TargetFramework>
+    <TargetFramework>netstandard2.0</TargetFramework>
     <NoWarn>$(NoWarn);CS1591</NoWarn>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <PackageTags>aspnetcore;proxy;headers;xforwarded</PackageTags>