Add 'group::dex' label as an unsupported group label

What does this MR do and why?

Fixing https://new-sentry.gitlab.net/organizations/gitlab/issues/366793/activity/?project=20.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (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