ci: use `macos-15`

[mkniewallner]

PR Checklist

• [x] A description of the changes is added to the description of this PR.
• [ ] If there is a related issue, make sure it is linked to this PR.
• [ ] If you've fixed a bug or added code that should be tested, add tests!
• [ ] If you've added or modified a feature, documentation in docs is updated

Description of changes

macOS 15 is now available in beta: https://github.com/actions/runner-images/issues/10686.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.1%. Comparing base (54380b3) to head (a42cad4). Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #873   +/-   ##
=====================================
  Coverage   92.1%   92.1%           
=====================================
  Files         37      37           
  Lines        992     992           
  Branches     175     175           
=====================================
  Hits         914     914           
  Misses        62      62           
  Partials      16      16           

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