14.0 release notes: Add warning about PG13

Why is this change being made?

A large ~"GitLab Premium" customer internally reported to be affected by DB problems after upgrading to from PG 11 to 13 during their GitLab upgrade from 13.12 to 14.0.

We have one warning about PG 13 in doc/administration/package_information/postgresql_versions.md, but that was only added a few versions later (gitlab-org/gitlab!70537 (merged)).

Related to gitlab-org/gitlab!77878 (merged).

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