Remove defunct macos-13 from CI

[IAlibay]

macos-13 (intel) is now dead

LLM / AI generated code disclosure

LLMs or other AI-powered tools (beyond simple IDE use cases) were used in this contribution: no

PR Checklist

• Issue raised/referenced?
• Tests updated/added?
• Documentation updated/added?
• package/CHANGELOG file updated?
• Is your name in package/AUTHORS? (If it is not, add it!)
• LLM/AI disclosure was updated.

Developers Certificate of Origin

I certify that I can submit this code contribution as described in the Developer Certificate of Origin, under the MDAnalysis LICENSE.

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📚 Documentation preview 📚: https://mdanalysis--5198.org.readthedocs.build/en/5198/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.71%. Comparing base (8ff4d09) to head (4affe09).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5198      +/-   ##
===========================================
- Coverage    92.72%   92.71%   -0.02%     
===========================================
  Files          180      180              
  Lines        22474    22474              
  Branches      3189     3189              
===========================================
- Hits         20840    20836       -4     
- Misses        1176     1179       +3     
- Partials       458      459       +1     

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