qt6: Don't set the HDPI attributes

They don't do anything on Qt6, but warn.

examples/dockwidgets/main.cpp

@@ -26,8 +26,10 @@ using namespace KDDockWidgets;
 
 int main(int argc, char **argv)
 {
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
     QApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
     QApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);
+#endif
     QApplication app(argc, argv);
 
     app.setOrganizationName(QStringLiteral("KDAB"));