Properly mark pg_stat_get_subscription() as returning a set.

 Properly mark pg_stat_get_subscription() as returning a set.
 
 The initial catalog data for this function failed to set proretset
 or provide a prorows estimate.  It accidentally worked anyway when
 invoked in the FROM clause, because the executor isn't too picky
 about this; but the planner didn't expect the function to return
 multiple rows, which could lead to bad plans.  Also the function
 would fail if invoked in the SELECT list.
 
 We can't easily back-patch this fix, but fortunately the bug's
 consequences aren't awful in most cases.  Getting this right is
 mainly an exercise in future-proofing.
 
 Discussion: https://postgr.es/m/1636062.1615141782@sss.pgh.pa.us