Improve Event specs

What does this MR do and why?

This updates the webhook payload fixture files and streamline/improve the Event specs.

Expected impact & dry-runs

No behavior change expected.

Action items

• (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