Propose PG minimum of 12.7 for GitLab 14 given new info

What does this MR do and why?

This MR updates the PostgreSQL requirements documentation to clarify that the minimum PostgreSQL version for GitLab 14 should be 12.7 rather than 12.10. This is based on new information learned. Here's a brief timeline:

• April 2022: The Recommend PostgreSQL version 12.10 (!84183 - merged) MR changed the guidance to say that 12.10 was required instead of 12.
• June 2022: The possible fixes section of the External PostgreSQL - Segmentation fault citing... (#364763 - closed) issue notes:

We initially believed 12.10 was the required version (GitLab.com and Omnibus deployments contradicted this evidence) but later confirmed that 12.7 has the fix.

See this comment.

This MR updates the PostgreSQL requirements documentation to account for what we learned.

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