Switch from macos-13 to macos-15-intel GitHub runner

[NicolaCourtier]

Description

Ahead of the deprecation of the macos-13 GitHub runner image, we can switch to macos-15-intel to keep running the unit tests on macOS with Intel CPU.

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #).

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: $ pre-commit run or $ nox -s pre-commit (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: nox -s tests
• The documentation builds: nox -s doctest
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.36%. Comparing base (c11f068) to head (ebc3c7d).
⚠️ Report is 38 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #830      +/-   ##
===========================================
- Coverage    89.81%   89.36%   -0.46%     
===========================================
  Files           61       63       +2     
  Lines         4783     4823      +40     
===========================================
+ Hits          4296     4310      +14     
- Misses         487      513      +26     

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