Bump Swift minimum to 6.0

[gwynne]

These changes are now available in 2.12.0

Also update CI and convert the tests to SwiftTesting.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.06%. Comparing base (cd47a70) to head (3958961).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #226      +/-   ##
==========================================
- Coverage   54.23%   54.06%   -0.18%     
==========================================
  Files           8        8              
  Lines         437      431       -6     
==========================================
- Hits          237      233       -4     
+ Misses        200      198       -2     

Files with missing lines	Coverage Δ
.../FluentPostgresDriver/PostgresError+Database.swift	92.30% <ø> (ø)

... and 3 files with indirect coverage changes

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• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[MahdiBM]

comment?

[gwynne]

To remind myself to make the code compatible with it later. It's not trivial, but should eventually be done.