Update cibuildwheel config to stop building Python 3.9 and free-threaded wheels

[anoadragon453]

This PR better documents how we configure cibuildwheel, and changes a couple things. The functional changes here:

• Moving cp39-* from skip_test to skip stops us from building wheels for Python 3.9. This was done incorrectly in Skip building Python 3.9 wheels with cibuildwheel #19119. That PR only stopped the tests from failing in CI, but we were still building (and shipping) cpython 3.9 wheels...
• Likewise moving cp3??t-* builds to skip prevent them from being created at all. Don't build free-threaded wheels #19140 only prevented testing them.

Otherwise:

• We don't build at all for i686 anyways, so -musllinux_1686 could be replaced by *i686* with no functional effect.

This PR is based on release-1.142, and I've marked it as a release blocker to stop Python 3.9 wheels from being built for the final 1.142.0 release (which claims that Python 3.9 is no longer supported).

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct (run the linters)

[anoadragon453]

These will no longer be required after cibuildwheel recognises our minimum defined python version in #19137.