releasing 2.21.2

[Jonas Hahnfeld]

Hi all,

with an initial CG update for GitLab committed, I'd like to propose
that we have our next unstable release 2.21.2 to get the documentation
available online.

Due to the problems we had in the past, I decided to test GUB before
Phil runs into problems. It took no less than 4 commits to make the
basics work: https://gitlab.com/lilypond/lilypond/-/merge_requests/148
Please review so that the fixes can go in soon-ish. Additionally, we
probably want a fix for issue #5999, proposed by Han-Wen in
https://gitlab.com/lilypond/lilypond/-/merge_requests/147
Anything else that must go in?

@Phil (assuming you want to continue doing the releases): You'll need
access to the GitLab repository to push release/unstable and eventually
merge to master. Please let me know your username so that I can add
you. I promise to help in any way I can in case something doesn't work
as expected for the new platform 😉

Cheers,
Jonas

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