Update for Swift 6.3 release

[gwynne]

These changes are now available in 4.9.0

Bump package minimum to 6.0, fix Concurrency warnings, standard cleanup stuff.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.60%. Comparing base (73529a6) to head (f43b2bd).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
- Coverage   82.02%   81.60%   -0.43%     
==========================================
  Files           7        7              
  Lines         217      212       -5     
==========================================
- Hits          178      173       -5     
  Misses         39       39              

Files with missing lines	Coverage Δ
...rces/FluentSQLiteDriver/FluentSQLiteDatabase.swift	86.04% <100.00%> (-0.77%)	⬇️

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