Fix new NIO warnings

[fabianfett]

This PR brings us up to speed with current Swift NIO main branch and fixes the warnings that will show up once a new NIO release is cut.

[codecov-commenter]

Codecov Report

Merging #300 (b73c4b6) into main (4b8ec14) will not change coverage. The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main     #300   +/-   ##
=======================================
  Coverage   43.91%   43.91%           
=======================================
  Files         115      115           
  Lines        9680     9680           
=======================================
  Hits         4251     4251           
  Misses       5429     5429           

Flag	Coverage Δ
unittests	43.91% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...es/PostgresNIO/Connection/PostgresConnection.swift	17.14% <0.00%> (ø)
Sources/PostgresNIO/Data/PostgresData.swift	6.89% <ø> (ø)
Sources/PostgresNIO/Data/PostgresRow.swift	49.24% <ø> (ø)
Sources/PostgresNIO/New/Messages/DataRow.swift	95.38% <ø> (ø)
Sources/PostgresNIO/New/PostgresCell.swift	96.77% <ø> (ø)