Upgrade pgx to v5.6.0 to fix nil arg handling

[shafi-fi9155]

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), but gorm.io/driver/postgres has not yet upgraded to this version.

[shafi-fi9155]

@jinzhu Any updates on this? We're blocked on upgrading from v1.4.5 to v1.4.6 or later.