Bump Swift minimum to 6.0

[gwynne]

These changes are now available in 2.15.0

Also update CI and convert the tests to SwiftTesting.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.05%. Comparing base (d2fd317) to head (9b38ce3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #275      +/-   ##
==========================================
+ Coverage   42.47%   43.05%   +0.57%     
==========================================
  Files          13       13              
  Lines         871      871              
==========================================
+ Hits          370      375       +5     
+ Misses        501      496       -5     

Files with missing lines	Coverage Δ
Sources/PostgresKit/PostgresDataTranslation.swift	67.27% <ø> (-0.37%)	⬇️

... and 3 files with indirect coverage changes

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[MahdiBM]

We should keep pg 13 I'd say, as long as Postgres themselves are supporting it.

[gwynne]

There were no protocol changes between 13 and 14, so I see no point in making the test matrix even larger; it doesn't actually tell you anything that the test for 14 doesn't.

[MahdiBM]

I wasnt thinking about a larger matrix. Mostly just replacing 14 with 13