Reverting change in source generators that require Roslyn 4.4 until we have a VS version with the compiler fix to consume them.

[joperezr]

When #74822 got merged we realized that the compiler that is now required for using the source generators is not yet present in any VS preview version yet (or a functioning SDK) so we are instead going to wait till after RC1 and 17.4 Preview 2 ships to make this change. Otherwise, we would need to disable all of the SDK tests that run against VS MSBuild, which is not worth the risk of doing.

cc: @jkoritzinsky @danmoseley @jaredpar @CyrusNajmabadi

[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.

[joperezr]

@danmoseley we would appreciate a sign off so we can merge and unblock the ingestion from runtime to installer. As the description says, we will do this once RC1 and 17.4 P2 ships.

[danmoseley]

approved (of course)

Do I understand correctly that with this reverted, we are using the polyfill. So we don't expect a perf issue, it's clumsy but shippable if we must -- is that right?

[joperezr]

Do I understand correctly that with this reverted, we are using the polyfill

Yes, this is a direct revert, and this is now falling back to use polyfill again. We weren't expecting a perf issue, on the other hand, we were expecting a perf gain without the polyfill, but given that late roslyn bug fix and the fact that it didn't make it to 17.4 preview 1 we will need to hold off on this.

So we don't expect a perf issue, it's clumsy but shippable if we must -- is that right?

Yes, this is shippable, but we would rather try again after RC1 ships for the same reasons we wanted to do it in the first place: we expect to see a significant perf increase, we want to build against newer compilers (which we have to do anyway), we want to use Roslyn APIs which are better tested than our polyfill approach.

[carlossanlop]

Approved, signed off, CI failure is unrelated (System.Net.Http.Functional.Tests, known).
Ready to merge.