Merge branch '5500-pg11-deprecation-warning' into 'master'

Recommend PostgreSQL 12 minimum in 13.6

See merge request gitlab-org/gitlab!46504

doc/install/requirements.md

@@ -148,6 +148,7 @@ 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).