Remove duo_chat_goal_command feature flag

What does this MR do and why?

Removes the duo_chat_goal_command feature flag, making the /goal slash command in Duo Agentic Chat generally available.

The flag has been enabled by default since 19.5 and the feature is deemed stable, so the gates it guarded can go:

• push_frontend_feature_flag(:duo_chat_goal_command, ...) in the EE gon helper
• the gate in getGoalFlow, which answered null with the feature off
• the flag entry for the goal plugin in the chat plugin registry
• the flag's YAML definition

The docs update for the same change is split out into !267402 (merged), which resolves #633466 (closed).

Closes #628226 (closed)

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