Disable deprecated Qt functions

Also Qt6 by default set these attributes to our preferred value.

src/app/main.cpp

@@ -134,7 +134,7 @@ int main(int argc, char *argv[])
     // We must save it here because QApplication constructor may change it
     bool isOneArg = (argc == 2);
 
-#if !defined(DISABLE_GUI)
+#if (QT_VERSION < QT_VERSION_CHECK(6, 0, 0)) && !defined(DISABLE_GUI)
     // Attribute Qt::AA_EnableHighDpiScaling must be set before QCoreApplication is created
     if (qgetenv("QT_ENABLE_HIGHDPI_SCALING").isEmpty() && qgetenv("QT_AUTO_SCREEN_SCALE_FACTOR").isEmpty())
         Application::setAttribute(Qt::AA_EnableHighDpiScaling, true);