Do not remove the pipeline:mr-approved label when no approvals

Context

• Follow-up of #1487 (closed)
• Closes #1507 (closed)

What does this MR do and why?

Do not remove the pipeline:mr-approved label from an MR: add it if there is at least one approval.

In the near future when we rollout pipeline tiers, we will be able to remove this processor entirely.

Expected impact & dry-runs

Once the pipeline:mr-approved is added to an MR, it will not be removed by triage-ops.

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

Closes #1507 (closed)