Refactor IncidentCreator to close the incident if it needs to

What does this MR do and why?

This is a follow-up to !2221 (comment 1384756191).

With this MR, I'm closing the incident using the API directly in IncidentCreator#execute, so that we get fresh data from the API directly, without having to set the incident's data manually or "fake" them.

Expected impact & dry-runs

The behavior would be exactly the same. I only simplify the internal implementation.

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