Adapt to QQuickWindow graphics backend API changes in Qt6

src/composite.cpp

@@ -310,7 +310,11 @@ bool Compositor::setupStart()
 
     if (!Workspace::self() && m_backend && m_backend->compositingType() == QPainterCompositing) {
         // Force Software QtQuick on first startup with QPainter.
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
         QQuickWindow::setSceneGraphBackend(QSGRendererInterface::Software);
+#else
+        QQuickWindow::setGraphicsApi(QSGRendererInterface::Software);
+#endif
     }
 
     Q_EMIT sceneCreated();