[release/7.0-preview6] Add annotations for ref fields to public API surface

[github-actions]

Backport of #71265 to release/7.0-preview6

/cc @AaronRobinsonMSFT

Customer Impact

This work adds new C# annotations to APIs that will be impacted in C# 11 (that is, ref fields). The goal here is for C# 11 to have the ref field support and for users to be able to use Preview 6 with the new language features. This ensures the Preview 6 ref pack is consumable with this language feature.

/cc @cston @stephentoub @jaredpar @jeffschwMSFT

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

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.

[jeffschwMSFT]

Approved. We will take for consideration in preview 6

[stephentoub]

Assuming this is enough to unblock the compiler being ingested for P6, LGTM. We'll revisit exactly where/how we want to use scoped in main for P7.

[jeffschwMSFT]

@carlossanlop this is ready when the branch is ready.