Recommend PostgreSQL 12 minimum in 13.6

- Add note to documentation indicating that in GitLab 13.6 the minimum
  recommended major version of PostgreSQL is 12

Related https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/5500Signed-off-by: Robert Marshall <rmarshall@gitlab.com>

doc/install/requirements.md

@@ -144,6 +144,7 @@ GitLab version | Minimum PostgreSQL version
 -|-
 10.0 | 9.6
 13.0 | 11
+13.6 | 12
 
 You must also ensure the `pg_trgm` and `btree_gist` extensions are [loaded into every
 GitLab database](postgresql_extensions.html).