ci: Update Xcode to 15.4

[cbaker6]

New Pull Request Checklist

• I am not disclosing a vulnerability.
• I am creating this PR in reference to an issue.

Issue Description

Update to Xcode 15.4 and the macOS 14 image which uses Apple Silicon by default.

Approach

• Install SwiftLint onto image as macOS 14 image doesn't contain this (unlike previous images)
• Remove enabling multiple cores as this setting was for Intel based images

TODOs before merging

• Verify visionOS builds and passes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.06%. Comparing base (077d457) to head (f93f696).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #166      +/-   ##
==========================================
- Coverage   91.21%   91.06%   -0.15%     
==========================================
  Files         178      178              
  Lines       12087    12054      -33     
==========================================
- Hits        11025    10977      -48     
- Misses       1062     1077      +15     

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[cbaker6]

visionOS build currently failing due to actions/runner-images#9511