Description

This PR makes some minor improvements to our River tests and DB migrations:

1. Speeds up our River tests. 2 tests in the upgrade_test worker were taking 20s and 40s because they were retrying a job and River's default retry policy is 1s->16s->1m21s, etc. See https://riverqueue.com/docs/job-retries. Now running go test ./internal/river -count=1 takes around 12s on my machine. Previously it was taking ~70s
2. When running the tests with verbose=true, one would see ERROR: database "jimm_test_testmigrationworker_error_6fgoa36q" is being accessed by other users. This was because we weren't commiting/rolling-back a tx we started with tx, err := sqlDb.Begin(). That's been fixed. I've also simplified the helper we use to create the test database.
3. I've removed handleDeprecatedMigrations which handled scenarios where a deployment was running a now very old version of JIMM that didn't use golang-migrate and then upgraded to a version of JIMM that did. We can be quite confident that no users are running this old release so the code can finally go. At the time we likely didn't have external users anyway and it was added out of an abundance of caution.

Engineering checklist

• Documentation updated
• Covered by unit tests
• Covered by integration tests