Lower "waiting for activity on tasks took longer than" log level.

It's perfectly normal to wait longer in several circumstances, and the
output can lead to spurious regression output changes.

src/backend/distributed/executor/multi_client_executor.c

@@ -881,7 +881,7 @@ MultiClientWait(WaitInfo *waitInfo)
 		}
 		else if (rc == 0)
 		{
-			ereport(DEBUG2,
+			ereport(DEBUG5,
 					(errmsg("waiting for activity on tasks took longer than %ld ms",
 							(long) RemoteTaskCheckInterval * 10)));
 		}