Update PostgreSQL versions: only minor when needed

What does this MR do and why?

This merge request updates the PostgreSQL requirements following a conversation on Slack.

• We should use PostgreSQL major versions where possible.
• With GitLab major version 14, PostgreSQL 12.10 is specifically required.
  • See !84183 (merged)
• With GitLab major version 15, we had written 12.0. I corrected this to 12 and did the same for planned GitLab major version 16.

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