Allow nested scoped group label

What does this MR do and why?

This MR fixes the bug reported in Repeated setting for ~"group::gitaly::git" (#1342 - closed)

We will allow nested scoped group label during the check for whether an MR has a valid group label. I think it makes sense to remove this restriction at least for group labels now, given that we have a few nested scoped group labels such as

// https://about.gitlab.com/groups.json 

"distribution_build": {
    "name": "Distribution::Build",
    "section": "enablement",
    "stage": "systems",
    "categories": [
      "build"
    ],
    "label": "group::distribution::build"
  },
  "distribution_deploy": {
    "name": "Distribution::Deploy",
    "section": "enablement",
    "stage": "systems",
    "categories": [
      "omnibus_package",
      "cloud_native_installation"
    ],
    "label": "group::distribution::deploy"
  },
  "gitaly_cluster": {
    "name": "Gitaly::Cluster",
    "section": "enablement",
    "stage": "systems",
    "categories": [
      "gitaly"
    ],
    "label": "group::gitaly::cluster"
  },
  "gitaly_git": {
    "name": "Gitaly::Git",
    "section": "enablement",
    "stage": "systems",
    "categories": [
      "gitaly"
    ],
    "label": "group::gitaly::git"
  },

closes: #1342 (closed)

Expected impact & dry-runs

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

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-policies-with-a-dry-run on how to perform dry-runs for new policies.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/doc/reactive/best_practices.md#use-the-sandbox-to-test-new-processors on how to make sure a new processor can be tested.

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