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

src/depot_gui.cpp

@@ -497,7 +497,7 @@ struct DepotWindow : Window {
 			switch (this->type) {
 				case VEH_TRAIN:
 					if (wagon) return MODE_ERROR;
-					FALLTHROUGH;
+					[[fallthrough]];
 
 				case VEH_ROAD:
 					if (xm <= this->flag_size.width) return MODE_START_STOP;