Don't use removed QApplication attribute

src/app/main.cpp

@@ -235,7 +235,7 @@ int main(int argc, char *argv[])
         // 3. https://bugreports.qt.io/browse/QTBUG-46015
 
         qputenv("QT_BEARER_POLL_TIMEOUT", QByteArray::number(-1));
-#if !defined(DISABLE_GUI)
+#if (QT_VERSION < QT_VERSION_CHECK(6, 0, 0)) && !defined(DISABLE_GUI)
         // this is the default in Qt6
         app->setAttribute(Qt::AA_DisableWindowContextHelpButton);
 #endif