Reduce the chance of duplicate fedramp nudge

What does this MR do and why?

This MR resolves 2 issues mentioned in Duplicate bot comments (#1338 - closed):

1. add auto_paginate to the milestones response, so it returns all of the milestones. Without it, it was only able to return 20 records so we ended up with an empty milestone entry in the comment.
2. Have this processor only reacting to label addition events, when the added labels are either severity or fedramp labels. This reduces the chance for the processor to be triggered, thus reducing duplicates. At most, this can only be duplicated two times, in rare possibilities.

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