Fix Fluent model encoding for SQLKit and several Sendable warnings

[gwynne]

These changes are now available in 1.50.0

Full change list:

• Require Swift 5.9
• Resolve several new Sendable-related warnings
• Fix an issue which caused unpredictable failures when encoding multiple Fluent models for SQLKit queries due to unstable column ordering.
• Fix NIO API (EmbeddedEventLoop) misuse in tests
• Disable unfixable flaky test

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 24.28%. Comparing base (2cceea2) to head (6378e3e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...s/FluentKit/Concurrency/Database+Concurrency.swift	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #624      +/-   ##
==========================================
- Coverage   24.43%   24.28%   -0.15%     
==========================================
  Files         149      149              
  Lines        9353     9349       -4     
==========================================
- Hits         2285     2270      -15     
- Misses       7068     7079      +11     

Files with missing lines	Coverage Δ
Sources/FluentKit/Model/Fields+Codable.swift	81.48% <ø> (-2.39%)	⬇️
Sources/FluentKit/Properties/Relation.swift	51.72% <ø> (ø)
...luentKit/Query/Builder/QueryBuilder+Paginate.swift	75.00% <ø> (ø)
Sources/FluentSQL/SQLDatabase+Model.swift	64.51% <100.00%> (ø)
...s/FluentKit/Concurrency/Database+Concurrency.swift	0.00% <0.00%> (ø)

... and 3 files with indirect coverage changes