add an explicit RTE_GROUP clause that does nothing (or falls through like VALUES/RESULT) for PG 18

Without it, an UPDATE … FROM (SELECT … GROUP BY …) on PG 18 will raise “Unrecognized range table entry.”

src/backend/distributed/planner/multi_router_planner.c

@@ -1027,6 +1027,9 @@ ModifyQuerySupported(Query *queryTree, Query *originalQuery, bool multiShardQuer
 		}
 		else if (rangeTableEntry->rtekind == RTE_VALUES ||
 				 rangeTableEntry->rtekind == RTE_RESULT
+#if PG_VERSION_NUM >= PG_VERSION_18
+				 || rangeTableEntry->rtekind == RTE_GROUP   /* Accept synthetic GROUP */
+#endif
 				 )
 		{
 			/* do nothing, this type is supported */