This PR improves the release checklist and scripts:
* Check that the release's commit hash is not all-numeric starting with
0 (this can break SemVer, which [required us to release
v4.21.0-rc2](https://github.com/leanprover/lean4/releases/tag/v4.21.0-rc2)).
* Check that projects being bumped to a release tag do not reference
`nightly-testing` anymore.
* Clarify how to create subsequent release candidates if an `-rc1`
already exists.
* Fix typos in the release checklist documentation.
This PR adds further automation to the release process, taking care of
tagging, and creating new `bump/v4.X.0` branches automatically, and
fixing some bugs.
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Co-authored-by: Johan Commelin <johan@commelin.net>
This PR fixes a number of bugs in the release automation scripts, adds a
script to merge tags into remote `stable` branches, and makes the main
`release_checklist.py` script give suggestions to call the
`merge_remote.py` and `release_steps.py` scripts when needed.
---------
Co-authored-by: Johan Commelin <johan@commelin.net>
