Replace magic constants used in pg_stat_get_replication_slot().

A few variables have been using 10 as a magic constant while
PG_STAT_GET_REPLICATION_SLOT_COLS can be used instead.

Author: Masahiko Sawada
Reviewed-By: Amit Kapila
Backpatch-through: 14, where it was introduced
Discussion: https://postgr.es/m/CAD21AoBvqODDfmD17DkEuPCvV2KbruukXQ2Vwrv5Xi-TsAsTJA@mail.gmail.com

src/backend/utils/adt/pgstatfuncs.c

@@ -2317,8 +2317,8 @@ pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
 	text	   *slotname_text = PG_GETARG_TEXT_P(0);
 	NameData	slotname;
 	TupleDesc	tupdesc;
-	Datum		values[10];
-	bool		nulls[10];
+	Datum		values[PG_STAT_GET_REPLICATION_SLOT_COLS];
+	bool		nulls[PG_STAT_GET_REPLICATION_SLOT_COLS];
 	PgStat_StatReplSlotEntry *slotent;
 	PgStat_StatReplSlotEntry allzero;