docs: gha example should use postgres 16 and Moodle 4.5 LTS

[jrchamp]

As Moodle main/5.2 requires postgres:16 and Moodle 4.5 is the current LTS, bump both values in the sample GitHub Action file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.21%. Comparing base (198ee6c) to head (7562bcf).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #381   +/-   ##
=========================================
  Coverage     88.21%   88.21%           
  Complexity      767      767           
=========================================
  Files            77       77           
  Lines          2334     2334           
=========================================
  Hits           2059     2059           
  Misses          275      275           

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