Enable Duo Chat for self-managed

What does this MR do and why?

Enable duo chat for self-managed.

Previously, DuoChat was available only for customers on gitlab.com instance.

With those changes we allow customers with ultimate license even on self-managed to access DuoChat and DuoChat only.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In rails console enable the experiment fully

   Feature.enable(:ai_duo_chat_switch)

2. In case you were working with ai features before - disable main AI feature flag

   Feature.disable(:ai_global_switch)

3. Make sure you have Anthropic token set up as described here: https://docs.gitlab.com/ee/development/ai_features/index.html#configure-anthropic-access
4. Run GDK with GITLAB_SIMULATE_SAAS=false option

solves https://gitlab.com/gitlab-org/gitlab/-/issues/434802