Add GitLab Duo Self-hosted Back to Staging-ref

Description

The GitLab Duo Self-Hosted menu used to show up in staging-ref but has recently disappeared. In the first place, staging-ref is a SaaS environment and we had to create a feature flag to enable self-hosted models (a self-managed feature) to appear in that environment.

This FF allowed the GitLab Duo nav item to appear in the admin menu where normally it wouldn't for a saas environment. This was fine until recently when Duo Workflow (saas) claimed that space and now overrides Duo Self-Hosted. This is not a problem in production since the code path will always be either self-managed or saas (and not both like we are trying to force in staging-ref).

Potential solutions:

• Update code logic to show Duo Self-hosted UI entrypoint alongside Duo Workflow. However it might be a bit weird/confusing/alarming to see those features together on the same page?
• Create a new menu item, for self-hosted Duo, using the same feature flag so it appears just for staging-ref.