Sendable cleanup

[gwynne]

These changes are now available in 2.14.0

SQLPostgresConfiguration is now Sendable, and Sendable warnings for the use of SQLDatabase.withSession(_:) have been fixed.

Additional changes:

• Swift minimum bumped to 5.9
• Several miscellaneous warnings fixed
• Cleanup of tests
• Minimal formatting pass on all code
• Update API docs theme settings

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 42.08%. Comparing base (99d09c4) to head (451d442).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/PostgresKit/PostgresDatabase+SQL.swift	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #271      +/-   ##
==========================================
+ Coverage   40.85%   42.08%   +1.22%     
==========================================
  Files          13       13              
  Lines         842      865      +23     
==========================================
+ Hits          344      364      +20     
- Misses        498      501       +3     

Files with missing lines	Coverage Δ
Sources/PostgresKit/ConnectionPool+Postgres.swift	37.50% <ø> (ø)
...stgresKit/Deprecations/PostgresConfiguration.swift	0.00% <ø> (ø)
...stgresConnectionSource+PostgresConfiguration.swift	0.00% <ø> (ø)
...Deprecations/PostgresDatabase+SQL+Deprecated.swift	0.00% <ø> (ø)
Sources/PostgresKit/PostgresConnectionSource.swift	100.00% <100.00%> (ø)
Sources/PostgresKit/PostgresDataTranslation.swift	67.51% <ø> (+0.47%)	⬆️
Sources/PostgresKit/PostgresRow+SQL.swift	80.00% <100.00%> (+1.42%)	⬆️
Sources/PostgresKit/SQLPostgresConfiguration.swift	85.10% <ø> (+0.66%)	⬆️
Sources/PostgresKit/PostgresDatabase+SQL.swift	74.57% <0.00%> (+1.84%)	⬆️

... and 2 files with indirect coverage changes

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[0xTim]

CI needs updating but otherwise LGTM