Drop support for postgresql 9.1 in GitLab 9.3

What does this MR do?

Documents that our minimum supported postgresql version is 9.2

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

Why was this MR needed?

9.1 is EOL and continuing to support it is an unnecessary technical burden

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #32851 (closed)