vm-builder: disable prepared statements support for pgbouncer (#589)
Disable prepared statement support for pgbouncer by setting `max_prepared_statements=0`. It was initially enabled in https://github.com/neondatabase/autoscaling/pull/566, but we encountered some problems with them. And we can't just revert that PR (to pgbouncer 1.18) because we started to use `auth_dbname`, which is added in pgbouncer 1.19. Ref https://neondb.slack.com/archives/C063B6SCHDX