fix: handle pg_replication_slots on pg<13

[michael-todorovic]

This PR fixes #1097 to handle pg < 13 for pg_replication_slots.
The original feat was #1027

[sysadmind]

I would prefer a more descriptive name for the query and I don't know that I love the way the scan uses a slice of any from a maintainability standpoint, but I think the code works as implemented and this can always be cleaned up later.

Thanks!