Release for v8.12 has started

Internals
1 
The release process for v8.12 will begin in about a week.  If you
have any new features that you want in and are relatively close to
to being done, now is a good time to do that.

Upcoming dates:
- 7th:  Branch day, merge window starts
- 15th: Merge window ends, testing starts
- 22nd: Testing ends
2 
The release process for v8.12 has begun: release checkpoints have
been created for all packages in the main distribution, and release
branches have been created when necessary. You can go on using
`master` branches as usual. The main Racket repo's is now bumped to
v8.12.0.1 (to avoid having two different trees with the same version).

If you have any bug-fixes and changes (including history updates)
that need to go in the release then make sure to specify that in the
commit message by using the word `merge` or `release` or mail the
relevant repo manager [1]) the commit SHA1s. Do not push commits
directly to `release` branches.

Please make sure that code that you're responsible for is as stable
as possible, and let me know if there is any new work that should
not be included in this release.

      >>> NOW IS THE TIME TO FIX BUGS THAT YOU KNOW ABOUT <<<

The time between now and the end of the merge window is for fixing
new errors that prevent proper functioning of major components and
that show up during the preparation for a release. You can also
finalize piece of work that is not yet complete, but please avoid
merging new features.

Note that nightly builds will go on as usual (starting from
v8.12.0.1 and going up as usual), and pre-release builds will be
available shortly at

  https://pre-release.racket-lang.org/


[1] https://github.com/racket/racket/wiki/release-repo-managers

Upcoming dates:
- 15th: Merge window ends, testing starts
- 22nd: Testing ends