CI: remove macos-13, add macos-15

[sk1p]

Contributor Checklist:

• I have added or updated my entry in the creators.json file
• I have added a changelog entry for my contribution
• I have added/updated documentation for all user-facing changes
• I have added/updated test cases

Reviewer Checklist:

• /azp run libertem.libertem-live-data passed

[sk1p]

/azp run libertem.libertem-live-data

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.10%. Comparing base (d35e858) to head (f1e4c0c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #187   +/-   ##
=======================================
  Coverage   75.10%   75.10%           
=======================================
  Files          32       32           
  Lines        2988     2988           
  Branches      351      351           
=======================================
  Hits         2244     2244           
  Misses        614      614           
  Partials      130      130           

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