Docs: update location of high availability page

Why is this change being made?

Update links to docs.gitlab.com/ee/administration/geo/replication/high_availability.md as it now lives at docs.gitlab.com/ee/administration/geo/replication/multiple-servers.md

Related to gitlab-org/gitlab#217825 (closed)

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the change relates to any part of the project other than updates to content and/or data files (e.g. team.yml) please make sure to ping @gl-static-site-editor in a comment for a review and merge.
  • If the DRI for the page/s being updated isn't immediate clear, then assign it to your manager.
  • 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

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