Docs: auto migrate should be false in all nodes during HA upgrade

What does this MR do?

Follow up from issue in support ticket (internal only: https://gitlab.zendesk.com/agent/tickets/142526). Without adding gitlab_rails['auto_migrate'] = false on the deploy node as well, they will fail on the "switching connection back to go through PgBouncer" step towards the end of the procedure.

Related issues

Additional details related to gitlab#33010 (closed)