Fix up FluentKit tests after SQLKit update

[gwynne]

These changes are now available in 1.47.3

Also bumps Swift minimum to 5.7 and adds ExistentialAny compliance.

[codecov]

Codecov Report

Attention: Patch coverage is 49.25000% with 203 lines in your changes are missing coverage. Please review.

Project coverage is 47.33%. Comparing base (5f0938a) to head (6030a72).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #599   +/-   ##
=======================================
  Coverage   47.33%   47.33%           
=======================================
  Files         106      106           
  Lines        4758     4758           
=======================================
  Hits         2252     2252           
  Misses       2506     2506           

Files	Coverage Δ
Sources/FluentKit/Database/Database.swift	77.77% <100.00%> (ø)
Sources/FluentKit/FluentError.swift	0.00% <ø> (ø)
Sources/FluentKit/Migration/Migration.swift	0.00% <ø> (ø)
Sources/FluentKit/Model/AnyModel.swift	73.07% <100.00%> (ø)
Sources/FluentKit/Model/Fields+Codable.swift	83.87% <100.00%> (ø)
Sources/FluentKit/Model/Fields.swift	97.43% <100.00%> (ø)
Sources/FluentKit/Model/Model.swift	68.75% <100.00%> (ø)
Sources/FluentKit/Properties/CompositeID.swift	88.46% <100.00%> (ø)
Sources/FluentKit/Properties/Property.swift	55.55% <ø> (ø)
Sources/FluentKit/Properties/Relation.swift	51.72% <100.00%> (ø)
... and 59 more