Get off of slonik fork

We are currently using a fork of `slonik`: https://github.com/alxndrsn/slonik/commit/b0299faa6405e474ea84d6c96dffecc98001a6e2

This is because of some stream-related issues.

At least one related patch has been accepted to upstream `node-postgres`: https://github.com/brianc/node-postgres/pull/2836

History of postgres clients used in Central:
* Started out with knex: chaining based query builder but wasn’t great to work with
* Streaming-related bugs
* Swapped to slonik because it seemed closer to desired interface
* Tests ran 30% slower than knex (not clear on production impact)
* Experienced several streaming-related bugs
* Single maintainer has come and gone in availability
* July 2022, started considering [`postgres.js`](https://github.com/porsager/postgres)
* Lots of energy behind it
* Uses a similar tag template literal interface as Slonik
* issa discovered after finished Slonik work
* Would have switched to it, but at the time, it didn’t support composing SQL fragments ("partials"), now introduced
* Transactions weren't initially working, at least in tests
* Initial PR: https://github.com/getodk/central-backend/pull/564
* Lots of tiny differences from Slonik. For the most part, it’s an improvement. A lot of things that we did by hand before, porsager postgres has an answer.
* porsager postgres has some weird quirks of its own though, e.g., no join utilities.
* So far it’s a code reduction. No queries so far have needed to change. Only core infrastructure has changed so far.
* Streaming issues - https://github.com/porsager/postgres/issues/455; would need to add support for multiple streams

A barrier to upgrading `slonik` has been several breaking changes since the version we were on. It seems there's likely a more deliberate approach to breaking changes now: https://github.com/gajus/slonik/issues/450#issuecomment-1409464447

There is now a bridge to use slonik on top of `postgres.js` instead of `node-postgres`: https://github.com/gajus/postgres-bridge