Update Core Team group in policies

We've recently moved the gitlab-core-team group under gitlab-org group, see relevant discussion (internal).

This will update the group in some triage label policies.

/cc @gitlab-org/gitlab-core-team

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 =>
  • Week-in-review