`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...

Start with bin/manage_users and inspect its client.query usage in relation to the reported node-postgres warning. Consult the linked node-postgres upgrade guide and verify the script's behavior; done means it no longer triggers the deprecated event-emitter warning.

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