Add warning if on <PG11

What does this MR do?

PG11 has been a formal dependency since 13.0 with deprecation warnings for <PG11 starting in 12.10 (e.g. https://about.gitlab.com/releases/2020/04/22/gitlab-12-10-released/#postgresql-11-is-now-the-default-version-of-postgresql-for-gitlab-self-managed and in other places).

We've seen development environments not upgraded to >= PG11 yet. This adds a warning to inform anybody on an unsupported version of Postgres to upgrade (with links and such).

This should be more helpful than seeing subtle behavioral issues or migrations fail in unclear ways.

Closes #224683 (closed)

Screenshots

For dev/test envs:

For other envs (no link to GDK instructions):

Does this MR meet the acceptance criteria?

Conformity

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