Release duo_chat_multi_thread to Self-Managed instances

What does this MR do and why?

Release duo_chat_multi_thread to Self-Managed instances

References

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Set GDK to be in Self-Managed mode (env.runit GITLAB_SIMULATE_SAAS=0)
2. Enable duo_chat_multi_thread flag in Rails console
3. Validate Duo Chat works as expected

MR acceptance checklist

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

Related to #519236 (closed)