Propagate MERGE ... WHEN NOT MATCHED BY SOURCE (#7807)

DESCRIPTION: Propagates MERGE ... WHEN NOT MATCHED BY SOURCE

It seems like there is not much needed to be done here.
`get_merge_query_def` from `ruleutils_17` is updated with "WHEN NOT
MATCHED BY SOURCE" therefore `deparse_shard_query` parses the merge
query for execution on the shard correctly.

Relevant PG commit:
postgres/postgres@0294df2f1

src/backend/distributed/planner/merge_planner.c

@@ -1546,8 +1546,8 @@ FetchAndValidateInsertVarIfExists(Oid targetRelationId, Query *query)
 			continue;
 		}
 
-		/* NOT MATCHED can have either INSERT or DO NOTHING */
-		if (action->commandType == CMD_NOTHING)
+		/* NOT MATCHED can have either INSERT, DO NOTHING or UPDATE(PG17) */
+		if (action->commandType == CMD_NOTHING || action->commandType == CMD_UPDATE)
 		{
 			return NULL;
 		}