[Humble] Fix: Update cmake version to 3.20

[mergify]

Backport of #295 to Humble.

[mergify]

Cherry-pick of 5837b89 has failed:

On branch mergify/bp/humble/pr-295
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 5837b89.
  (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:   clearpath_common/CMakeLists.txt
	modified:   clearpath_control/CMakeLists.txt
	modified:   clearpath_customization/CMakeLists.txt
	modified:   clearpath_customization/project_bringup/CMakeLists.txt
	modified:   clearpath_customization/project_description/CMakeLists.txt
	modified:   clearpath_description/CMakeLists.txt
	modified:   clearpath_generator_common/CMakeLists.txt
	modified:   clearpath_manipulators/CMakeLists.txt
	modified:   clearpath_manipulators_description/CMakeLists.txt
	modified:   clearpath_mounts_description/CMakeLists.txt
	modified:   clearpath_platform_description/CMakeLists.txt
	modified:   clearpath_sensors_description/CMakeLists.txt

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   .github/workflows/ci.yml
	deleted by us:   clearpath_diagnostics/CMakeLists.txt

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