Sourcebuild 7.0: scoped ref for latest Roslyn

[rainersigwald]

This change is required because of new C# features interacting with ref captures.

Per @jaredpar:

How ref capture works is a core problem we've been struggling with in the ref field design. The initial design went too far into the "let everything be captured" space, that lead to a number of compat breaks so we tweaked on aspect severely in the other direction, and after more refinement ended up in a more explainable place. Essentially any ref can be captured in the return of the method or out parameters. In this case it's a ctor, it has a ref so that can be captured but you don't intend that here so scoped to tell us it can't happen.

This is required to remove a 7.0 sourcebuild patch.

[jaredpar]

Finally leaving the 2019's? 😄