XML Element[fullscreen], child of element[gui], not defined in SDF.

## Environment
* Ubuntu 22.04
* Source build, Gazebo Garden

SDFormat spec defines an element `` in ``.
However specifying this in an SDF (see example below) yields an error message:
```
[/sdf/world[@name="fullscreen_test"]/gui/fullscreen:/home/johan/model.sdf:L6]: XML Element[fullscreen], child of element[gui], not defined in SDF. Copying[fullscreen] as children of [gui].
```

It seems that [not all GUI functionality is implemented](https://github.com/gazebosim/sdformat/blob/3fdc0a1fe7026fbc1f004de018daf83839a2914e/src/Gui.cc#L67), yet the strange thing is that on first sight [the fullscreen element _is_](https://github.com/gazebosim/sdformat/blob/3fdc0a1fe7026fbc1f004de018daf83839a2914e/src/Gui.cc#L60-L61).
So I assume it is rather an issue with the [parser](https://github.com/gazebosim/sdformat/blob/sdf13/src/parser.cc)?

On a side note: the `` element doesn't seem to do anything either...

Corresponding SDF file:
```

true

true

https://fuel.gazebosim.org/1.0/OpenRobotics/models/Ground Plane

```