`bin/manage_users` drops deprecation warning for future node-postgres versions

Currently, I get the following warning when using the `bin/manage_users` script:

> (node:78) DeprecationWarning: Using the automatically created return value from client.query as an event emitter is deprecated and will be removed in pg@7.0. Please see the upgrade guide at https://node-postgres.com/guides/upgrading
> Using password from commandline...