Add severity labels with add only scoped feature

What does this MR do and why?

This MR adds the severity label as part of a new add only scoped labels feature. This allows scoped labels to be used with the label command but does not allow them to replace existing scoped labels.

Currently when wider community contributors add labels during triage they cannot use the scope of severity labels despite this being documented in the handbook: https://handbook.gitlab.com/handbook/engineering/infrastructure/engineering-productivity/issue-triage/#partial-triage-checklist

See this recent example of an issue where a community contributor attempted to use @gitlab-bot label ~"severity::4" but received an invalid message: gitlab-org/gitlab#457543 (comment 1876428520)

Describe in detail what your merge request does and why.

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