Add support for UR18 (backport #1524)

[mergify]

This requires UniversalRobots/Universal_Robots_ROS2_Description#340.

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This is an automatic backport of pull request #1524 done by Mergify.

[mergify]

Cherry-pick of 233d524 has failed:

On branch mergify/bp/humble/pr-1524
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 233d524.
  (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:   README.md
	new file:   ur_robot_driver/config/ur18_update_rate.yaml
	modified:   ur_robot_driver/doc/usage/startup.rst
	new file:   ur_robot_driver/launch/ur18.launch.py
	modified:   ur_robot_driver/launch/ur_control.launch.py
	modified:   ur_robot_driver/test/test_common.py

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   ur_moveit_config/launch/ur_moveit.launch.py
	deleted by us:   ur_robot_driver/launch/ur_rsp.launch.py
	deleted by us:   ur_robot_driver/urdf/ur.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