Fix pg_upgrade to detect invalid logical replication slots on PG19

Hi Hackers,

The PG19-optimized slot catchup query uses a CTE that filters on
invalidation_reason IS NULL, then cross-joins it with the main slot
query.  When ALL logical slots in a database are invalid, the CTE
returns zero rows, and the cross join produces an empty result set.
This causes pg_upgrade to silently skip those slots entirely --
neither detecting them as invalid (which should block the upgrade)
nor attempting to migrate them.

The pre-PG19 query path does not have this problem because it queries
pg_replication_slots directly without a cross join. This may not impact
upgrade to PG19 but will change the behavior for PG20 upgrade.

Fix by changing the cross join to a LEFT JOIN,
so that invalid slots still appear in the result set with NULL
caught_up values.

Regards,
Lakshmi