Fix OID passed to object-alter hook during ALTER CONSTRAINT

The OID of the constraint is used instead of the OID of the trigger --
an easy mistake to make.  Apparently the object-alter hooks are not very
well tested :-(

Backpatch to 12, where this typo was introduced by 578b2297

Discussion: https://postgr.es/m/20210503231633.GA6994@alvherre.pgsql

src/backend/commands/tablecmds.c

@@ -10319,7 +10319,7 @@ ATExecAlterConstraint(Relation rel, AlterTableCmd *cmd,
 			copy_tg->tginitdeferred = cmdcon->initdeferred;
 			CatalogTupleUpdate(tgrel, &copyTuple->t_self, copyTuple);
 
-			InvokeObjectPostAlterHook(TriggerRelationId, currcon->oid, 0);
+			InvokeObjectPostAlterHook(TriggerRelationId, tgform->oid, 0);
 
 			heap_freetuple(copyTuple);
 		}