Following guidance in https://github.com/LCAS/aoc_distro/wiki/Migration-to-new-Gazebo implement the transition from Gazebo classic to Gazebo Sim for the hunter_gazebo. Some changes may be required in other ROS2 packages in this repository, but mostly it is about changes to hunter_gazebo.

Whereever possible retain compatibility with Gazebo Classic, and keep the changes small and manageable.