Drop Swift 6.0

[fabianfett]

• Drop Swift 6.0 support
• Add 6.3 CI

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.74%. Comparing base (f294b62) to head (d1f5e5f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #642      +/-   ##
==========================================
- Coverage   76.75%   76.74%   -0.01%     
==========================================
  Files         135      135              
  Lines       10391    10391              
==========================================
- Hits         7976     7975       -1     
- Misses       2415     2416       +1     

see 3 files with indirect coverage changes

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