571816 Duo Chat Usage Description

What does this MR do and why?

This change updates the description text for an AI analytics metric. The description previously referred to "AI Code Suggestions" but has been corrected to say "GitLab Duo Chat" to better reflect what the metric actually measures - user engagement with GitLab's chat-based AI feature rather than code suggestions.

See the current description under #571816 (closed)

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Before	After

How to set up and validate locally

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Related to #571816 (closed)