Setup Geo site for Staging Ref

Per discussion in #350741 (comment 817084975), we need to configure Staging Ref EU site using Geo:

• Architecture: 3k Omnibus environment
• Location: EU (since existing Staging Ref instance is in US)
• Use file based replication or our object storage replication per #350741 (comment 817213265)

Steps:

• ✅ Request hostname request for geo.staging-ref.gitlab.com - https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/15266
• ✅ Prepare GET configs for Staging Ref EU following Advanced - Geo docs in GET - https://gitlab.com/gitlab-org/quality/gitlab-environment-toolkit-configs/staging-ref/-/merge_requests/66
• ✅ Adjust CI config for Staging Ref to update Geo instance, ideally dogfooding near-zero downtime updates (related to #350741 (comment 817201544))
• Configure Unified URL - https://gitlab.com/gitlab-org/quality/gitlab-environment-toolkit-configs/staging-ref/-/merge_requests/84 - blocked - moved to #370312
• Create follow up to increase deployment speed