Avoid long-living cran-* branches

[krlmlr]

The best way to resolve merge conflicts is to never have them occur in the first place.

Sketch:

• init_release() also takes the responsibility of pre_release(), making sure that the commit that opens the PR has [ci skip] to avoid computing for naught
• pre_release() goes away
• release() creates a tag and merges the cran-* branch into main
  • The fledge GHA workflow avoids updating the main branch if a release is pending (PR exists)
• New reinit_release() for restarting a release attempt based on a tag, same procedure with release()
• post_release() must work off of the most recent non-9000 tag, and can be run unconditionally

An easy win is the tweak to the fledge GHA workflow. Will do this first and ponder on the other details later.

[krlmlr]

Released RMariaDB with a local verson of ledge that solves the first three bullet points. Need to wrap up and implement reinit_release() (as needed) and automated creation of release.