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.

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.
To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.