Add reason to keep postgresql 9.2 for CI

What does this MR do?

Explains the reason why not to use postgresql to 9.6 for CI tests.

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

As PostgreSQL 9.2 was already deprecated and with omnibus packages we are shipping GitLab with PostgreSQL 9.6, so we should indicate the reason why we are using PostgreSQL 9.2 for CI, instead of 9.6.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• [na/] Tests added for this feature/bug
• Review
  • [n/a] Has been reviewed by UX
  • [n/a] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [n/a] Internationalization required/considered

What are the relevant issue numbers?

#41749 (closed)