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.
54dd7aae8c