replace gz_gui_vendor with gz-gui8

[alsora]

rosdistro build failed with

Traceback (most recent call last):
  File "/tmp/ros_buildfarm/ros_buildfarm/scripts/devel/create_devel_task_generator.py", line 287, in resolve_names
    resolved_names = resolve_for_os(
  File "/usr/lib/python3/dist-packages/rosdep2/catkin_support.py", line 85, in resolve_for_os
    d = view.lookup(rosdep_key)
  File "/usr/lib/python3/dist-packages/rosdep2/lookup.py", line 204, in lookup
    return self.rosdep_defs[rosdep_name]
KeyError: 'gz_gui_vendor'

I think that there are two errors here:

• it shouldn't be listed as a build tool
• we should depend on gz-gui8