CI: clean up wheel build workarounds now that Cython 3.1.0 is out

[rgommers]

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

This simplifiers wheel builds for free-threaded CPython, which is useful in itself and I thought was a good idea before backporting the Windows cp313t wheel support to the 2.3.x branch, as just discussed at #61249 (comment).

This should have no changes in behavior, and mostly reverts the workarounds for unreleased Cython added initially in gh-60146. Other notes:

• Changes the free-threaded-support cibuildwheel setting to enable, because the former is now deprecated.
• This leaves the license concatenation behavior unchanged. Note that it's skipped on Windows, and happens on other platforms. This was added without any discussion in CI: Add Windows wheels for the free-threaded build #60146. It looks inconsistent, but the bash invocation doesn't work on Windows so I'd like to leave it unchanged in this PR. A useful follow-up PR may be to remove the ad-hoc concatenation in favor of starting to use PEP 639.

I ran the wheel builds on my fork before opening this PR, they're passing (CI logs).

[lithomas1]

Thanks Ralf!

[mroeschke]

Thanks @rgommers

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout 2.3.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 17f0dd6233a881702b36a301f4b8dd82f7d9f9a8

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #61446: CI: clean up wheel build workarounds now that Cython 3.1.0 is out'

4. Push to a named branch:

git push YOURFORK 2.3.x:auto-backport-of-pr-61446-on-2.3.x

5. Create a PR against branch 2.3.x, I would have named this PR:

"Backport PR #61446 on branch 2.3.x (CI: clean up wheel build workarounds now that Cython 3.1.0 is out)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.