doc: Fix up setGraphicsApi() since tag

It's the [set]sceneGraphBackend() functions that are from 5.8, the new
functions in Qt 6 are since 6.0.

Pick-to: 6.5 6.4 6.2
Change-Id: If36f42615d53bd964f05ba42846b9322d8f1a0e4
Reviewed-by: Christian Strømme <christian.stromme@qt.io>

src/quick/items/qquickwindow.cpp

@@ -3880,7 +3880,7 @@ QSGRendererInterface *QQuickWindow::rendererInterface() const
     graphics API based on the platform and other conditions, set \a api to
     QSGRendererInterface::Unknown.
 
-    \since 5.8
+    \since 6.0
  */
 void QQuickWindow::setGraphicsApi(QSGRendererInterface::GraphicsApi api)
 {