Add details for upgrading external postgres from 12 to 13

Existing documentation points to the RHEL docs for steps to upgrade external DB.
However, some steps need to be tweaked.

JIRA:
https://issues.redhat.com/browse/SAT-37171

Author: Akshay Gadhave

guides/common/modules/proc_upgrading-postgresql-on-the-external-database-system.adoc

@@ -10,7 +10,15 @@ Upgrade the external database to PostgreSQL 13
 endif::[]
 
 .Procedure
+. Backup the `var/lib/pgsql/data/postgresql.conf` and `/var/lib/pgsql/data/pg_hba.conf` files.
 . Follow the instructions in {RHELDocsBaseURL}8/html/deploying_different_types_of_servers/using-databases#fast-upgrade-using-the-pg_upgrade-tool_migrating-to-a-rhel-8-version-of-postgresql[Fast upgrade using the pg_upgrade utility] in _{RHEL}{nbsp}8 Deploying different types of servers_ to upgrade the external database.
+Instead of enabling the `postgresql:13` module, switch to the module:
++
+[options="nowrap", subs="+quotes,verbatim,attributes"]
+----
+# yum module switch-to postgresql:13
+----
+. Restore the `var/lib/pgsql/data/postgresql.conf` and `/var/lib/pgsql/data/pg_hba.conf` files from the backup.
 ifdef::katello,satellite,orcharhino[]
 . Correct the permissions on the `evr` extension on the external database:
 +