Update Threat Insights assign-refinement.yml for bugs only

What does this MR do and why?

Describe in detail what your merge request does and why.

Previously we auto-assigned feature issues that were both scheduled and in workflowrefinement. This causes issues as feature work is generally aligned to overarching epics and subject matter expertise.

We intentionally removed bugs from auto-assignment via !2446 (merged), for better control over planning and predictablity.

Our team now schedules typebug into refinement the milestone prior to when we expect to work on it. As bugs are more generic in nature, it's reasonable to have them auto-assigned for refinement. This also allows for an even distribution of refinement efforts amongst the team. A refinement bug issue can still be manually assigned to an engineer when there is a preference.

This change additionally allows both backend and frontend bugs in refinement/schedule to be auto-assigned. It was previously just backend.

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