Codechange: Replace old non-standard attributes with C++17/20 standard attributes.

src/vehicle.cpp

@@ -1391,7 +1391,7 @@ bool Vehicle::HandleBreakdown()
 			SetWindowDirty(WC_VEHICLE_VIEW, this->index);
 			SetWindowDirty(WC_VEHICLE_DETAILS, this->index);
 
-			FALLTHROUGH;
+			[[fallthrough]];
 		case 1:
 			/* Aircraft breakdowns end only when arriving at the airport */
 			if (this->type == VEH_AIRCRAFT) return false;