Upgrade pgx to v5.6.0 to fix nil arg handling

## Description
gorm.io/driver/postgres currently depends on pgx v5.5.5, which has a breaking change in how nil values are handled. For example previously, nil values for jsonB columns were converted to {} via Valuer.Value(), but in pgx v5, nil is now inserted as NULL, causing NOT NULL constraint violations.

This issue was addressed in pgx v5.6.0 ([jackc/pgx#1367](https://github.com/jackc/pgx/issues/1367)), but gorm.io/driver/postgres has not yet upgraded to this version.