CI: Upgrade to ubuntu-24.04, install Python free threading from conda-forge
Seems like that the python-freethreading from conda-forge is not recognized as a valid Python, no idea why. But other than that this looks great, much cleaner. Probably needs a bump of meson/meson-python. submitted https://github.com/cython/cython/issues/6870 for the warning. thanks @mroeschke  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 b2b2d04e419e44932d51017ececb5c3a86b15925
```

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

```
git commit -am 'Backport PR #61410: CI: Upgrade to ubuntu-24.04, install Python free threading from conda-forge'
```

4. Push to a named branch:

```
git push YOURFORK 2.3.x:auto-backport-of-pr-61410-on-2.3.x
```

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

> "Backport PR #61410 on branch 2.3.x (CI: Upgrade to ubuntu-24.04, install Python free threading from conda-forge)"

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](https://github.com/MeeseeksBox/MeeseeksDev).
                 The free threading stuff isn't on 2.3 so no need for backport