Upgrade post - Geo moves wikis to SSF

Why is this change being made?

Wikis have been moved into the Geo self-service framework. While this requires no action from the users, it is important that we highlight this.

There will be a FF that will allow customers to switch back to the old mode of replication/verification in case they encounter issues.

We will eventually remove the FF altogether once we are confident the changes are working well in the field.

This upgrade post relates to feature issue gitlab-org/gitlab#367925 (closed)

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
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  • 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.

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