remove a few occurrences of "qt5"

There are still a lot!

GraphicsView/demo/Circular_kernel_2/Circular_kernel_2.cpp

@@ -336,7 +336,7 @@ int main(int argc, char **argv)
   app.setOrganizationName("GeometryFactory");
   app.setApplicationName("Circular_kernel_2 demo");
 
-  // Import resources from libCGAL (Qt5).
+  // Import resources from libCGAL (Qt6).
   CGAL_QT_INIT_RESOURCES;
 
   MainWindow mainWindow;