Remove all code that solely existed to support Swift 5.8

[fabianfett]

Land after #515.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.24%. Comparing base (c13a11a) to head (ecdae88).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #516   +/-   ##
=======================================
  Coverage   60.24%   60.24%           
=======================================
  Files         125      125           
  Lines        9191     9191           
=======================================
  Hits         5537     5537           
  Misses       3654     3654           

Files with missing lines	Coverage Δ
Sources/ConnectionPoolModule/ConnectionPool.swift	94.39% <ø> (ø)
Sources/PostgresNIO/New/NotificationListener.swift	67.53% <ø> (ø)
Sources/PostgresNIO/New/VariadicGenerics.swift	93.24% <ø> (ø)

... and 2 files with indirect coverage changes