Upgrade macOS Runner Image to macos-13

[mbsantiago]

GitHub recently deprecated the macos-12 runner image, which was used to build the bundled Whombat executable for macOS.

To ensure maximum compatibility with macOS versions, we should use the oldest available runner image. Executables bundled on an older macOS version are generally forward-compatible with newer releases.

This PR updates the workflow to use the macos-13 runner image for building the macOS executable.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.47%. Comparing base (155020c) to head (340a5e0).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #46   +/-   ##
=======================================
  Coverage   66.47%   66.47%           
=======================================
  Files         179      179           
  Lines        8367     8367           
=======================================
  Hits         5562     5562           
  Misses       2805     2805           

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