pg_replication_slots scrape broken in pg 12

[ringerc]

https://github.com/prometheus-community/postgres_exporter/blob/a4ac0e674769805a9b282edb9a7791e64cd977e3/collector/pg_replication_slot.go#L95C1-L96C13

These columns are missing in Pg 12 so this scraper will be broken on this postgres version.

See #1027