Offboard departing GC members and onboard new GC members

[danielgblanco]

• Update the community-members.md and and codeowners: https://github.com/open-telemetry/community/
• For each new GC member, complete an issue created with the GC Member Onboarding template.
• For each departing GC member:
  • Encourage them to join #opentelemetry-gc-alum
  • Revoke access to the systems listed the GC Member Onboarding issue.

See #1494 for past actions.

[danielgblanco]

I think it'd be nice to have a GC member offboarding issue template with a some tasks to complete. It will basically be an inverse of the onboarding, but simpler.