Clarify label-missed-resources policy purpose

What does this MR do and why?

Describe in detail what your merge request does and why.

Follow up on !1979 (comment 1282164368)

I realized that the TRIAGE_MOVE_MILESTONE_FORWARD variable does not actually work when reviewing !1979 (merged), and that pointed me to https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/.gitlab/ci/hygiene.yml#L164, and this is when I found out about Move milestone while we add missed labels (!367 - merged) which combined the moving milestone forward automation with the missed-resources policy.

Now I see that for other groups, the moving milestone forward automation is actually configured in https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/.gitlab/ci/missed-resources.yml

This Merge request will remove any occurrence of the TRIAGE_MOVE_MILESTONE_FORWARD variable, and at the same time clarify the purpose of https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/policies/stages/hygiene/missed-resources.yml by giving it a more relevant rule name.

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-with-a-dry-run on how to perform dry-runs.

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