roachtest: replace lib/pq with pgx

Lib/pq is no longer actively maintained; it's also quite limited in its features around connection management and alleged less reliable [1] than pgx. Specifically, there isn't a robust way to track down the root cause of test failures like [2],

```
read tcp 172.17.0.3:59624 -> 3.16.78.36:26257: i/o timeout
```

Pgx supports multiple ips [3], which could help with retries. Some day it will also support client-side load-balancing [4]. See the internal slack thread [5] for more context.

[1] https://github.com/cockroachdb/cockroach/issues/143121#issuecomment-2739835367
[2] https://github.com/cockroachdb/cockroach/issues/142789
[3] https://www.postgresql.org/docs/11/libpq-connect.html#LIBPQ-MULTIPLE-HOSTS
[4] https://github.com/jackc/pgx/issues/2059
[5] https://cockroachlabs.slack.com/archives/C023S0V4YEB/p1742465527376359

Jira issue: CRDB-48841

The issue names roachtest but no files, tests, or entry points; start by locating its lib/pq imports and the roachtests that exercise those connections. Validate the replacement with the affected roachtest commands, including the connection and retry behavior described in the issue.

Written by the indexing model from the issue text.