[postgresql_server] Could do with some upgrade documentation

[Alphix]

The postgresql_server role could do with some information about how to do an upgrade...e.g. from bullseye (version 13) to bookworm (version 15).

In essence, the steps I had to were (writing this down so I don't forget it):

1. Do a normal distro upgrade
2. apt install postgresql-contrib
3. pg_dropcluster 15 main --stop
4. pg_upgradecluster 13 main
5. apt purge postgresql-13 postgresql-client-13
6. edit /etc/ansible/facts.d/postgres.fact and manually change the version (13 -> 15)
7. re-run the role on the controller

I'll see if I get around to adding something like this to the docs...