Ensure Patroni detects PosgtreSQL upgrade

The initial implementation of Patroni in !3984 (merged) is only tested against PostgreSQL 11. When PostgreSQL 12 is shipped, the upgraded procedure with gitlab-ctl pg-upgrade must be verified with Patroni. Currently it fails.

We must ensure that:

• pg-upgrade works on Patroni nodes
• The procedure for upgrading a cluster is documented
• Patroni can detect the upgrade and the difference between the running version and the upgraded version (postgresql::enable recipe already does this and warns the user if there is a difference) (the upgrade procedure restarts the service and this is not relevant anymore).