Add rule to remove ux-missing-labels when severity exists

What does this MR do and why?

Describe in detail what your merge request does and why.

Adds a new automation:ux-missing-labels rule to remove that label when a ~severity::* label exists

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

259 issues will have automation:ux-missing-labels removed: https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/2200795513

• 2 severity1 - https://gitlab.com/groups/gitlab-org/-/issues?sort=created_date&state=all&label_name[]=automation:ux-missing-labels&label_name[]=severity::1
• 9 severity2 - https://gitlab.com/groups/gitlab-org/-/issues?sort=created_date&state=all&label_name[]=automation:ux-missing-labels&label_name[]=severity::2
• 38 severity3 - https://gitlab.com/groups/gitlab-org/-/issues?sort=created_date&state=all&label_name[]=automation:ux-missing-labels&label_name[]=severity::3
• 210 severity4 - https://gitlab.com/groups/gitlab-org/-/issues?sort=created_date&state=all&label_name[]=automation:ux-missing-labels&label_name[]=severity::4

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review