Wrong joint name: "cast_wheel_rear_link" - should be "cast_wheel_rear_joint"

[slgrobotics]

Wrong joint name: "cast_wheel_rear_link" at Tools/simulation/gz/models/rover_differential/model.sdf causes a warning:

INFO  [init] Spawning Gazebo model
Warning [Utils.cc:115] Non-unique name[cast_wheel_rear_link] detected 2 times in XML children of model with name[rover_differential].Warning [Utils.cc:115] Joint with name [cast_wheel_rear_link] in model with name [rover_differential] has a name collision, changing joint name to [cast_wheel_rear_link_joint].INFO  [gz_bridge] world: lawn, model: rover_differential_0

The fix (line 328):

    <joint name='cast_wheel_rear_joint' type='ball'>
      <pose relative_to='cast_wheel_rear_link'/>
      <parent>base_link</parent>
      <child>cast_wheel_rear_link</child>
    </joint>

BTW, how about 10 kg mass at line 8 for more realistic behavior? ;-)

@chfriedrich98