Introduce working group for remaining GitHost.io to GitLab.com migrations

Why is this change being made?

This MR introduces a new working group focused on migrating 4 remaining customers off of GitHost and on to GitLab.com. The ongoing work will require coordination of two specific tracks:

• Development: in order to unblock these migrations successfully (especially the migration of the largest and most complex customer off of GitHost), capabilities and quality of migration tools must improve.
• Migration: as development unblocks migration steps for these customers, ProServ should execute on migration plans, relaying back findings and results.

Currently, there is no regular and ongoing channel of communication to coordinate these tasks and an execution path to migration has not been formally defined.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what