SGG: Roll out per-region Slack handles for all global groups

After a successful test within the Ginkgo group (see #4297 (closed)), we now want to roll this out for all SGGs. Aside from the biggest Access Request of all times, this also requires adding steps to Onboarding and Offboarding to make sure that the groups keep in sync after the initial setup. We also need to account for the possibility of someone changing SGGs.

Onboarding

I'm not sure at what point SGG are currently introduced during Onboarding. Assuming the target SGG for a new starter is already known at that time, it could be added to the role based AR template: https://gitlab.com/gitlab-com/team-member-epics/access-requests/-/blob/master/.gitlab/issue_templates/role_baseline_access_request_tasks/department_customer_support/role_support_engineer.md

I have created a branch with the change, but not an MR yet, as I wasn't sure if there's some specific requirements or concerns with the use of "variables" like SGGNAME or REGION.

Offboarding

Not sure where/how that is handled. Would the Slack account not be decomissioned completely anyway?

Transferring between SGGs

When a team member changes SGG, they also need to be moved.

Access Request for Initial Setup

I will create an AR similar to the Ginkgo one, but for all remaining groups.