ci: Use macOS 26 Tahoe with apple-clang 21

[mathbunnyru]

High Level Overview of Change

Would be great to use Apple Clang 21 instead of old 17.

Context of Change

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.0%. Comparing base (0711a7b) to head (43ecba5).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #7601     +/-   ##
=========================================
- Coverage     82.0%   82.0%   -0.0%     
=========================================
  Files         1007    1007             
  Lines        76890   76890             
  Branches      8976    8971      -5     
=========================================
- Hits         63049   63044      -5     
- Misses       13832   13837      +5     
  Partials         9       9             

see 3 files with indirect coverage changes

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