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

src/network/network_content_gui.cpp

@@ -894,7 +894,7 @@ public:
 						return ES_HANDLED;
 					}
 					/* space is pressed and filter is focused. */
-					FALLTHROUGH;
+					[[fallthrough]];
 
 				default:
 					return ES_NOT_HANDLED;