Fix creation of the database enum when using a non-default space

[gwynne]

Previously, if a non-nil jobsTableSpace was provided to the JobModelMigration when using PostgreSQL, the job state enum type was incorrectly created in the default schema instead of alongside the jobs table. This is now fixed, and the name of the space is no longer used to prefix the names of the enumeration or the index on the table. The migration's revert(_:) method will attempt to drop the old name from the default space (using IF EXISTS) in order to maintain compatibility.

Additional changes:

• Package.swift no longer imports Foundation to get at the current environment (I learned that Context.environment exists).
• The minimum version requirements for the dependencies have been bumped.
• The CI has been updated to include tests against the latest PostgreSQL and MySQL versions.
• A number of issues in the README have been fixed and information on the driver's configurable options has been added.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.23%. Comparing base (61b2fa9) to head (23c29b1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
+ Coverage   98.18%   98.23%   +0.05%     
==========================================
  Files           7        7              
  Lines         275      283       +8     
==========================================
+ Hits          270      278       +8     
  Misses          5        5              

Files with missing lines	Coverage Δ
Sources/QueuesFluentDriver/JobModelMigrate.swift	100.00% <100.00%> (ø)
...ources/QueuesFluentDriver/SQLKit+Convenience.swift	98.33% <100.00%> (ø)

🚀 New features to boost your workflow:

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