[NEOS-1118] Replace uses of pgx for stdlib for Postgres (in most cases)

[nickzelei]

Using pgx is fine for backend queries that talk to our database. However, any postgres query that is used by the worker should just use stdlib. I think this will simplify a lot of our logic as there are many places where we are using both pgx and database/sql due to using mysql. This will be exasperated as we continue to add support for other databases.

This will be a larger refactor, but it will pay off. It could even be worth exploring pgx's stdlib feature where you can register pgx to be used under the hood of the stdlib, so we can continue to use database/sql everywhere, but utilize pgx underneath so that we can have a single, standardized interface everywhere.

_{From SyncLinear.com | NEOS-1118}