Add Postgres 18 to tests

[jklaise]

With Postgres 18 just released thought it would be good to add it to the tests.

[TkTech]

The setup action has been updated with v18 support so we'll officially support v18 with release 0.26.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (25_1@510580f). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             25_1      #55   +/-   ##
=======================================
  Coverage        ?   69.16%           
=======================================
  Files           ?       56           
  Lines           ?     2938           
  Branches        ?        0           
=======================================
  Hits            ?     2032           
  Misses          ?      906           
  Partials        ?        0           

Flag	Coverage Δ
unittests	69.16% <ø> (?)

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