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

src/network/network_gui.cpp

@@ -2134,7 +2134,7 @@ struct NetworkJoinStatusWindow : Window {
 							progress = 15; // We don't have the final size yet; the server is still compressing!
 							break;
 						}
-						FALLTHROUGH;
+						[[fallthrough]];
 
 					default: // Waiting is 15%, so the resting receivement of map is maximum 70%
 						progress = 15 + _network_join_bytes * (100 - 15) / _network_join_bytes_total;
@@ -2307,7 +2307,7 @@ struct NetworkCompanyPasswordWindow : public Window {
 		switch (widget) {
 			case WID_NCP_OK:
 				this->OnOk();
-				FALLTHROUGH;
+				[[fallthrough]];
 
 			case WID_NCP_CANCEL:
 				this->Close();