Add support for UR18 (backport #118)

[mergify]

This requires UniversalRobots/Universal_Robots_ROS2_Description#340.

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This is an automatic backport of pull request #118 done by Mergify.

[mergify]

Cherry-pick of afc7048 has failed:

On branch mergify/bp/humble/pr-118
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit afc7048.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   ur_simulation_gz/launch/ur_sim_control.launch.py
	modified:   ur_simulation_gz/launch/ur_sim_moveit.launch.py
	modified:   ur_simulation_gz/test/test_gz.py

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   ur_simulation_gz/test/test_description.py
	deleted by us:   ur_simulation_gz/urdf/ur_gz.urdf.xacro

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally