Documentation about how to upgrade postgresql images

See: https://issues.redhat.com/browse/RHPAM-4464

We need to add the following procedure in the documentation when the operator is upgraded from 7.13.0 (Postgres 10) 7.13.1(Postgres 12) and 7.13.2 (next version with postgresql 13)

Upgrade from postgres 10 to postgres 12 https://github.com/sclorg/postgresql-container/tree/master/12#upgrading-database-by-switching-to-newer-postgresql-image-version

Upgrade from 12 to 13 https://github.com/sclorg/postgresql-container/tree/master/13#upgrading-database-by-switching-to-newer-postgresql-image-version