Update docs for PostgreSQL 12 clustering

Patroni is the only option for PostgreSQL 12 clustering since repmgr
will no longer supported for new versions.

doc/administration/postgresql/index.md

@@ -13,7 +13,7 @@ There are essentially three setups to choose from.
 This setup is for when you have installed GitLab using the
 [Omnibus GitLab **Enterprise Edition** (EE) package](https://about.gitlab.com/install/?version=ee).
 
-All the tools that are needed like PostgreSQL, PgBouncer, Repmgr are bundled in
+All the tools that are needed like PostgreSQL, PgBouncer, Patroni, and repmgr are bundled in
 the package, so you can it to set up the whole PostgreSQL infrastructure (primary, replica).
 
 [> Read how to set up PostgreSQL replication and failover using Omnibus GitLab](replication_and_failover.md)

doc/administration/postgresql/replication_and_failover.md

@@ -29,6 +29,11 @@ You also need to take into consideration the underlying network topology, making
 sure you have redundant connectivity between all Database and GitLab instances
 to avoid the network becoming a single point of failure.
 
+NOTE: **Note:**
+As of GitLab 13.3, PostgreSQL 12 is shipped with Omnibus GitLab. Clustering for PostgreSQL 12 is only supported with
+Patroni. See the [Patroni](#patroni) section for further details. The support for repmgr will not be extended beyond
+PostgreSQL 11.
+
 ### Database node
 
 Each database node runs three services: