Go dev guidelines: Recommend jackc/pgx over lib/pq

What does this MR do?

So far we've been recommending the lib/pq driver for interacting with PostgreSQL databases in Go applications. This guideline seems to be in place for (at least) almost two years (commit).

About 7 months ago, the lib/pq maintainer added a deprecation notice to the project readme, stating that "this package is effectively in maintenance mode and is not actively developed. Small patches and features are only rarely reviewed and merged. We recommend using pgx which is actively maintained." Most of the issues open since then have been left unanswered.

The pgx driver can also be used with the standard database/sql package, so replacing pq with pgx on an existing application is a matter of swapping _ "github.com/lib/pq" for _ "github.com/jackc/pgx/v4/stdlib" and renaming the driverName for sql.Open from postgres to pgx.

Apart from the extra features and performance improvements provided by the custom pgx interface, it's also possible to rely on some of these functionalities while using its stdlib interface, like logging and callbacks, which can be configured through pgconn.Config and pgx.ConnConfig, as demonstrated in the docs.

This MR proposes that we start recommending jackc/pgx over lib/pq.

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