Consider setting CheckEolTargetFramework=false globally

[crummel]

These warnings almost always hit source-build a release cycle before they hit the contributing repos due to our N->N rebuild testing. While upgrading TFMs is something we want, we can detect issues with non-current TFMs using prebuilt detection, so removing this warning (which is usually made an error) shouldn't make source-build more dangerous.

[MichaelSimons]

[Triage] In the VMR 9 timeframe we should consider removing this global setting so that the product usages of the EOL TFMs are forced to be fixed. However in the meantime when source-build is not the official build, this is a source of breaks that require last minute patches with no value so disabling it globally seems like the right decision.

[ViktorHofer]

The switch is still set globally for all VMR projects: https://github.com/dotnet/installer/blob/c01b62c0fcfdfd7af0c1af9a476faa8756bf913d/src/SourceBuild/content/repo-projects/Directory.Build.props#L123-L124