Port to harmonic

[luca-della-vedova]

New feature implementation

Implemented feature

Depends on open-rmf/rmf_traffic_editor#483.
This PR ports the codebase to Harmonic and drops support for Gazebo classic.
Currently blocked on 24.04 being released and available in the buildfarm, as well as Harmonic rosdep keys being released.

Implementation description

Several parts were cleaned up:

• rmf_demos_maps now only needs to build one map, which should speed up its build by a fair bit.
• gz_classic plugins and references have been removed.
• ignition renamed to gazebo.
• TPE support has been dropped.
• Default version for gazebo bumped from 6 (fortress) to 8 (harmonic).

[mxgrey]

Tested this against 24.04 and rolling. I just needed to tweak the package manifest to correct some dependencies. Now this seems to work great.

Now we just need to do some minor fixes to open-rmf/rmf_simulation#114 and ideally sort out the CI situation, then we can simultaneously merge all the related PRs.

[mxgrey]

There seems to be a CI issue with ignition-cmake2. My immediate guess is that we're not using the correct dependency key in one or more of the package.xml files for rmf_simulation.

[luca-della-vedova]

There seems to be a CI issue with ignition-cmake2. My immediate guess is that we're not using the correct dependency key in one or more of the package.xml files for rmf_simulation.

rmf_simulation won't really work on rolling / noble until its harmonic migration PR is merged, so this is a bit of a chicken and egg problem.
I'll merge it now and trigger a rerun here, that should fix the CI.

[mxgrey]

I just realized that the CI issue here is probably caused by the current main of rmf_simulation being incompatible with rolling + noble. I suppose if we get open-rmf/rmf_simulation#114 working and merged then this PR's CI should be sorted out too.

[luca-della-vedova]

I added a not too pretty workaround to suppress an unused variable test failure in 2727480. It's not great but it's also what the rclpy examples do to deal with the same issue