Migrate to Garden

[ahcorde]

Signed-off-by: ahcorde ahcorde@gmail.com

Migrated to Gazebo Garden

[luca-della-vedova]

It's a bit hard to make a case for moving to Garden for RMF right now, we are currently sticking with Fortress for a few reasons:

• LTS support, Fortress being an LTS release.
• Side by side installation: Fortress can still be installed side to side (binary) with Gazebo classic, the moment we migrate to Garden that capability would be lost since they conflict and I'm not sure what would happen when rosdep tries to install dependencies on repos that depend on both (such as this one). In RMF we currently install both and give users the choice of what they want to use at runtime.
• ros_ign binary availability: the package for Humble is available in binaries only targeting Fortress, if we decided to use Garden we would have to add yet another source build package in the workspace.

I'm happy to get started on the ign -> gz migration by merging in all the changes that are still backward compatible with Fortress (i.e. renaming if headers are available?), but would hold full migration at least until there is a new LTS and probably only when we decide to stop supporting Gazebo Classic.

[luca-della-vedova]

With #114 we now migrated to Harmonic, as well as finished the rename