Update GitLab Duo usage metrics to display absolute user counts

What does this MR do and why?

AI Summary

This change updates the GitLab Duo and SDLC trends dashboard to track user counts instead of usage rates for several Duo features. Previously, the system calculated percentages showing how many users with AI access actually used features like Code Suggestions, Duo Chat, and Root Cause Analysis. Now it simply shows the raw number of users who used each feature, removing the need to calculate rates against total assigned users. The labels and descriptions were also simplified - for example, "Duo Chat usage" became "Chat (non-agentic) usage" and "Duo Code Review requests" became just "Code Review requests". This makes the metrics more straightforward by focusing on actual usage numbers rather than adoption percentages.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Please make sure that you're on the GitLab Ultimate plan (if necessary, request a license).
2. Configure your GDK to use ClickHouse
3. Make sure ClickHouse is being used for Analytics by running this in the Rails console: Gitlab::CurrentSettings.current_application_settings.update(use_clickhouse_for_analytics: true)
4. Create a new project or select an existing one.
5. Seed data for the project using its ID in PROJECT_ID=STEP_4_PROJECT_ID FILTER=ai_usage_stats bundle exec rake db:seed_fu
6. Navigate to Analytics Dashboard > GitLab Duo and SDLC trends in the sidebar menu
7. Verify that the updated usage metrics work correctly

MR acceptance checklist

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Related to #589605 (closed)