Recommend PostgreSQL 12 minimum in 13.6

What does this MR do?

Updates documentation to state minimum recommended PostgreSQL version is now 12 in 13.6

Does this MR meet the acceptance criteria?

Yes, scheduled by Product and Engineering.

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Related omnibus-gitlab#5500 (closed)