Port PackageValidation flaky build failure fixes to 6.0

[dagood]

• Update compatibility package and unpin compiler version runtime#60847
• Improve loading of Roslyn assemblies on validate package task #22277

This would help source-build reliability in particular (dotnet/source-build#2501) and allow removing this workaround:

• [release/6.0] Disable package validation in source-build for reliability runtime#60881

[ericstj]

@safern @dagood can this be closed, or have these not yet been ported to 6.0? Adjusting to 6.0.2xx

[dagood]

Seems like it can be closed. @lbussell @MichaelSimons correct me if I'm wrong.

[ericstj]

FWIW I don't see @safern's fixes in 6.0 codebase.

[dagood]

Ah, yep, I must have misread some PRs.