Update GitHub Actions to Xcode 26.3

[MortenGregersen]

Summary

• update CI matrix Xcode version from 26.1 to 26.3
• update iOS simulator destination OS from 26.1 to 26.3

Testing

• not run locally

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.10%. Comparing base (f8ff497) to head (70daa9a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
+ Coverage   74.00%   74.10%   +0.10%     
==========================================
  Files          23       23              
  Lines         950      950              
==========================================
+ Hits          703      704       +1     
+ Misses        247      246       -1     

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