Release process should include explicit beta test period

[kinnison]

We should update the release process to include an explicit beta testing process where the staged release is offered up to users to validate that no regressions have been introduced. Since there are a lot of release process items tied to pushing to the stable branch, this will involve:

• Agreeing the subset of binaries which will be included in this beta process
• Designating a beta branch for this part of the release
• Providing for the beta branch to upload to dev-static.rlo
• Adjusting the release process to use the beta branch, and to report and request testing on internals.rust-lang.org
• Adjusting the release process to include how we move from a beta release to a stable release

There may be further changes also needed, but that's the bare minimum.

Once some thought has gone into this, we will likely need to consult with the release team.

[rami3l]

Since this has been the new norm since #3719, I think we can safely close this issue now.