Stop making src modifications to every repo's global.json

[MichaelSimons]

When building the VMR, there are several checked in files that are being modified. This yields a poor UX as these must be undone before you can build cleanly again. These edits will also become a nuisance in .NET 9 when the VMR can be used to checkin changes.

One type of file being modified is each repo's global.json. The repo's global.json is updated to use the dotnet version specified in the VMR's global.json

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[mmitche]

Also related: #3781. See costings there.