Update group-definition.yml

What does this MR do and why?

To account for Fulfillment PM changes

Alex --> remove from Purchase, replace with Courtney. Alex to become Utilization PM.

Tatyana --> added to platform

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