Configure Staging Ref to use Unified URL

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As part of discussion about Geo sites on Staging Ref, unified URL should be configured.

The work is currently blocked by Support Geo Unified URL for Cloud Native Hybrid... (gitlab-environment-toolkit#733 - closed)

WIP MR for Staging Ref to update config once the above issue is closed: https://gitlab.com/gitlab-org/quality/gitlab-environment-toolkit-configs/staging-ref/-/merge_requests/84 - blocked