Manage mr-approved label when MR is updated

Context

Closes #1487 (closed).

Follow-up of !2690 (merged) and !2694 (merged).

What does this MR do and why?

When testing locally (I finally fixed my GDK 🎉), I noticed that when the product resets approvals are reset (e.g. when enabling this project setting), we don't send a merge_request.approval/merge_request.approved webhook event. Instead, we send a merge_request.update.

This MR is here to ensure that we act on those events as well to add/remove the pipeline:mr-approved label.

It's not perfect though, as I also noticed that it will take a few seconds (at least locally), to actually remove the approvals, and the event is fired right away, so triage-ops checks the MR, and the approval is still present.

On another commit or MR update though, the label will eventually be removed, so I think that's a good-enough tradeoff.

Screenshots

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