Upgrade post - Geo ssh push proxy workflow change

Why is this change being made?

This is an upgrade post related to Geo.

In 16.1 Geo changes the workflow for git push operations against secondary sites. The new workflow will leverage GitLab shell to proxy the push request made against a secondary site to the primary site.

This change will remove 60 second timeout imposed by puma previously. However, the new workflow does have a 1.86GB limit on the size of the push request. We aim to remove this limit in a future release.

Related epic:gitlab-org&8819

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

---