AI impact analytics: Rename `Duo seats: Assigned and used` panel

What does this MR do and why?

Rename the Duo seats: Assigned and used panel to Duo Chat: Unique users to be consistent with the description.

MR acceptance checklist

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

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Before	After

How to set up and validate locally

1. Ensure that you are using GitLab Ultimate
2. Configure your GDK to use ClickHouse

# enable clickhouse
Feature.enable(:clickhouse_data_collection)
Feature.enable(:event_sync_worker_for_click_house)

# enable the worker to sync code suggestions events to clickhouse
Feature.enable(:code_suggestion_events_in_click_house)

3. Seed AI analytics data: FILTER=ai_usage_stats bundle exec rake db:seed_fu
4. Create a new group
5. Navigate to the Analytics dashboard page for the new group (ex. http://gdk.test:3000/groups/flightjs/-/analytics/dashboards)
6. Select the AI impact analytics dashboard

Related to #468304 (closed)