Add AI checkbox for community contributions

What does this MR do and why?

We are now allowed to accept AI generated contributions, but we need to ask the contributor to indicate such.

We do not want to add further noise with all of the existing bots etc, so we are silently adding a checkbox to the MR descrription.

See gitlab-org/developer-relations/contributor-success/team-task#437

Expected impact & dry-runs

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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