sagemathgh-36338: CI: Merge open blocker PRs in all CI workflows + other improvements
    
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The new script `.ci/merge-fixes.sh` uses the GitHub CLI to retrieve open
PRs with the "blocker" label from sagemath/sage and tries to merge them
into the tested branch, ignoring any PRs that give merge failures.

We call this script in all CI workflows. Thus,
- hotfixes for the linter (such as sagemath#36327) or
- fixes for build problems (such as sagemath#36276)

will take effect in the CI workflows of all PRs.

This can be seen in the runs of the workflows of the present PR (section
"Merge CI fixes from sagemath/sage").

Additionally,
- we fix `debian-trixie`, which failed because a system package is no
longer available
- we fix `macos-conda-maximal` by removing some fictitious
distros/conda.txt files
- we refactor ci-macos using a new reusable workflow; example runs:
   - https://github.com/mkoeppe/sage/actions/runs/6308124398
   -
https://github.com/sagemath/cysignals/actions/runs/6316714387?pr=185
- we update the ci-macos platforms, including a configuration for the
new and problematic Xcode 15.0

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- [ ] I have created tests covering the changes.
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### ⌛ Dependencies

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URL: sagemath#36338
Reported by: Matthias Köppe
Reviewer(s): Dima Pasechnik, Jonathan Kliem, Kwankyu Lee, Matthias Köppe, Tobias Diez