Update minimum Swift version

[gwynne]

These changes are now available in 4.13.0

Now requires Swift 6, and swift-format was run on everything. Converted the tests to SwiftTesting, too.

[codecov]

Codecov Report

❌ Patch coverage is 32.00000% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.21%. Comparing base (223b27d) to head (458dc86).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/Fluent/ModelAuthenticatable.swift	0.00%	8 Missing ⚠️
Sources/Fluent/ModelTokenAuthenticatable.swift	0.00%	8 Missing ⚠️
Sources/Fluent/FluentProvider.swift	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #794      +/-   ##
==========================================
- Coverage   60.42%   60.21%   -0.22%     
==========================================
  Files          15       15              
  Lines         374      372       -2     
==========================================
- Hits          226      224       -2     
  Misses        148      148              

Files with missing lines	Coverage Δ
...luent/Concurrency/FluentProvider+Concurrency.swift	0.00% <ø> (ø)
.../ModelCredentialsAuthenticatable+Concurrency.swift	85.71% <ø> (ø)
...es/Fluent/Concurrency/Pagination+Concurrency.swift	100.00% <100.00%> (ø)
...rces/Fluent/Concurrency/Sessions+Concurrency.swift	0.00% <ø> (ø)
Sources/Fluent/Fluent+Cache.swift	68.75% <100.00%> (-1.25%)	⬇️
Sources/Fluent/Fluent+History.swift	88.88% <ø> (ø)
Sources/Fluent/Fluent+Paginate.swift	100.00% <ø> (ø)
Sources/Fluent/Fluent+Pagination.swift	100.00% <ø> (ø)
Sources/Fluent/Fluent+Sessions.swift	77.41% <100.00%> (ø)
Sources/Fluent/MigrateCommand.swift	8.33% <ø> (ø)
... and 5 more

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

[MahdiBM]

Looks good.