MAINT: Update to macos-15-intel

[larsoner]

macos-13 is deprecated, so update to macos-15-intel for the x86_64 runner.

Since MNE-BIDS doesn't do any heavy linalg work or anything, I'd actually prefer just to drop macos-15-intel and assume upstream libs (MNE, SciPy, etc.) properly test support generalization on those archs. Thoughts?

Merge reqs will need to be updated, I'm happy to do that and merge once the above is discussed ☝️

[larsoner]

... actually I realized a nice compromise is to move macos-15-intel out of the full build matrix and into one special-cased include entry. That reduces CI burden but still does some testing on that arch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.41%. Comparing base (2adc50c) to head (17826c9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1460   +/-   ##
=======================================
  Coverage   97.41%   97.41%           
=======================================
  Files          41       41           
  Lines        9371     9371           
=======================================
  Hits         9129     9129           
  Misses        242      242           

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[larsoner]

Ahh CircleCI is down. I'll adjust merge reqs and merge manually