Reduce PostgreSQL max_connections from 3000 to 300
Now that we're using pgbouncer we definitely don't need a max_connections
of 3000, thus we should reduce this. A value of 300 should be enough given that pgbouncer allows for up to 150 connections.
Changing this requires downtime, and we must first reduce this on the primary (and restart it) before applying this to the secondaries; otherwise they will refuse to start. This also requires waiting for a brief moment between applying this on the primary and secondaries, as the secondaries need to catch up.