Rebrand and update for 5.0.1 (#27406)

* Move to 5.0.1 versions

* Move to GA .NET SDK
- required for some tests to pass

* Remove feeds that won't be needed after 5.0.0 is GA

* Cherry-pick `$(CrossgenOutput)` and Directory.Build.*.in changes from master
- [master] Update dependencies from dotnet/runtime dotnet/efcore (#26788)
    - 219ecd68
- when project template tests run test assets, need latest shared Fx bits
- hit `BadImageFormatException`s otherwise
    - test projects build against uploaded packages
    - those packages reference the 5.0.1 shared framework
    - the ASP.NET parts of the 5.0.1 shared Fx are `crossgen`ed to target Windows

- Use runtime and ref/ assemblies matching repo in Helix testing
    - add Directory.Build.*.in files based on project template test infrastructure
    - use files as import boundary where the project doesn't create its own Directory.Build.* files
    - ensure `dotnet-watch` tests also use the latest runtime and ref/ assemblies

- Extend Helix Directory.Build.* workarounds
    - generate Directory.Build.* files when restoring any projects
    - include generated files in Helix runs needing the latest runtime
    - copy generated files when testing `dotnet-watch` locally
    - include generated content in Microsoft.NET.Sdk.BlazorWebAssembly.IntegrationTests test assets
        - remove duplicate settings from existing Directory.Build.* files
    - ensure shared framework and targeting packs are laid out under .dotnet/ before test assets restore

- Disable `crossgen` when building for Helix runs
    - make `$(CrossgenOutput)` property override-able
    - use override in CI jobs that submit to other platforms
        - for now, leave the ARM64 Helix jobs alone (build on Ubuntu, run in Debian)

* Correct an `$(IsTestAssetProject)` setting
- affected projects are all test assets or provide test support
- without this, a number of the projects are incorrectly marked as shipping

* Baseline released 5.0.0 packages
- this is a complete rewrite of eng/Baseline.xml
  - based on the 5.0.0 MergedManifest.xml file

* Add 5.0.0 PackageOverrides.txt and PlatformManifest.txt files
- need consistent versions when servicing targeting packs

Co-authored-by: dotnet-maestro[bot] <42748379+dotnet-maestro[bot]@users.noreply.github.com>