Update RepoTasks to use the FrameworkReference workaround from GenerateFiles

[wtgodbe]

Today, we can't update RepoTasks.csproj's TFM until we get an SDK with the new TFM baked in. Our other projects can update before that because they get the workaround from GenerateFiles:

aspnetcore/eng/tools/GenerateFiles/Directory.Build.targets.in

Lines 10 to 27 in b7ee505

	<!-- Workaround when there is no vNext SDK available; copy known apphost/framework reference info from 6.0. -->
	<ItemGroup>
	<KnownAppHostPack
	Include="@(KnownAppHostPack->WithMetadataValue('TargetFramework', 'net6.0'))"
	TargetFramework="${DefaultNetCoreTargetFramework}"
	Condition="'@(KnownAppHostPack->Count())' != '0' AND
	!(@(KnownAppHostPack->AnyHaveMetadataValue('TargetFramework', '${DefaultNetCoreTargetFramework}')))" />
	<KnownRuntimePack
	Include="@(KnownRuntimePack->WithMetadataValue('TargetFramework', 'net6.0'))"
	TargetFramework="${DefaultNetCoreTargetFramework}"
	Condition="'@(KnownRuntimePack->Count())' != '0' AND
	!(@(KnownRuntimePack->AnyHaveMetadataValue('TargetFramework', '${DefaultNetCoreTargetFramework}')))" />
	<KnownFrameworkReference
	Include="@(KnownFrameworkReference->WithMetadataValue('TargetFramework', 'net6.0'))"
	TargetFramework="${DefaultNetCoreTargetFramework}"
	Condition="'@(KnownFrameworkReference->Count())' != '0' AND
	!(@(KnownFrameworkReference->AnyHaveMetadataValue('TargetFramework', '${DefaultNetCoreTargetFramework}')))" />
	</ItemGroup>

. This blocks source build because we wind up with mixed TFM assets in the source build payload in dotnet/installer.

[ghost]

Thanks for contacting us.

We're moving this issue to the .NET 8 Planning milestone for future evaluation / consideration. We would like to keep this around to collect more feedback, which can help us with prioritizing this work. We will re-evaluate this issue, during our next planning meeting(s).
If we later determine, that the issue has no community involvement, or it's very rare and low-impact issue, we will close it - so that the team can focus on more important and high impact issues.
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