presenterplugin: Only call setClearBeforeRendering in Qt5

As documented in https://doc.qt.io/qt-6/quick-changes-qt6.html, this
should not be needed 1in Qt6 and was consequently removed.

plugins/presenter/presenterplugin.cpp

@@ -34,7 +34,9 @@ public:
         windowFlags |= Qt::WindowTransparentForInput;
 #endif
         setFlags(windowFlags);
+#if QT_VERSION_MAJOR < 6
         setClearBeforeRendering(true);
+#endif
         setColor(QColor(Qt::transparent));
 
         setResizeMode(QQuickView::SizeViewToRootObject);