Replace TargetFrameworks with TargetFramework where possible

[jkotas]

No description provided.

[dotnet-issue-labeler]

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

[jkotas]

Follow up on #66175 (comment)

[danmoseley]

@ViktorHofer didn't you have some concurrent or recent change to change TargetFrameworks to TargetFramework in some way? Just want to make sure this doesn't conflict or have some wierd consequence.

[jkotas]

The test failures are #66232 and infrastructure issue (Failed to upload results).

[ViktorHofer]

@ViktorHofer didn't you have some concurrent or recent change to change TargetFrameworks to TargetFramework in some way? Just want to make sure this doesn't conflict or have some wierd consequence.

Note that there was a technical limitation that required all libraries projects to use the TargetFrameworks property even if the project wasn't multi-targeting but that was fixed with 20e8f90. Reference and source projects were updated via the same commit and test projects were then later updated with 7671b50.

The src/libraries projects touched here were either added/updated meanwhile, or my regex skills that I used to update the projects failed me 🤦 Anyway, the change looks good to me.