Add check for v19.0 mattermost breaking change

Adding a new check

This check tests for omnibus installations using Mattermost. This is deprecated in version 19.0

https://gitlab.com/gitlab-com/Product/-/work_items/14319

Closes #222 (closed)

Verification steps for review

1. Enable Mattermost in the environment. This can be done with a fake url.
   1. Edit gitlab.rb to add mattermost_external_url 'https://mattermost.example.com``'``
   2. Run gitlab-ctil reconfigure
   3. Confirm Mattermost is runner gitlab-ctl status should show a mattermost service.
2. Run the frontend and verify the check shows as changed.
3. Remove / comment out the mattermost url in gitlab.rb and reconfigure.
4. Run gitlab-ctl status again to confirm the service is no longer running.
5. Run the frontend again and observe the check returns ok.

Author checklist

• After opening the MR:
  • Set it to the current milestone
  • Ask the Maintainer from the Reviewer roulette suggestion for review

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Reviewer checklist

• I followed the verification steps and confirm the functionality of the new check
  • I executed the check as presented in this MR by running the generated execution image (registry.gitlab.com/gitlab-com/support/toolbox/gitlab-detective:MR-MRIID-PIPELINEID) with the frontend
  • The check output executed
• This check does only perform read operations
• This check does not output more than necessary on stdout for the check to function
• The message explains what it means when this check does not pass
• The workaround_url provides actionable information/steps for affected users
  • Consider if a Knowledge Base article should exist to serve as the ideal workaround URL
• This check is not using the Rails console/runner, or has Maintainer approval for doing so
• This check is not using a Rake task, or has Maintainer approval for doing so
• If this is a breaking change check:
  • It has the corresponding xx_breaking_changes tag (xx being the major release version for the change)
  • The workaround_url goes to the entry on the https://docs.gitlab.com/update/deprecations/ page
  • The ref_url goes to the deprecation issue linked from that entry
  • The title is the same as that entry
  • The version_started is equal to the announcement_milestone of the deprecation
  • The version_fixed is equal to the removal_milestone of the deprecation