Draft: Enable agentic_chat_ga by default

What does this MR do and why?

agentic_chat_ga was originally going to be replaced by ai_duo_agent_platform_ga_rollout, however we found that ai_duo_agent_platform_ga_rollout is only used under SaaS setting, thus under self-managed ai_duo_agent_platform_ga_rollout will stay as disabled and cannot be used for our FE purposes

This MR enables the agentic_chat_ga by default to release it to self-managed instances. This means the agentic_chat_ga flag only serves for FE purposes purely.

This does not affect gitlab.com since we have explicitly disabled it on production already.

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Related to #562720