collab: Restrict usage of the LLM service to accounts older than 30 days (#16133)

This PR restricts usage of the LLM service to accounts older than 30
days.

We now store the GitHub user's `created_at` timestamp to check the
GitHub account age. If this is not set—which it won't be for existing
users—then we use the `created_at` timestamp in the Zed database.

Release Notes:

- N/A

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Co-authored-by: Max <max@zed.dev>