Remove `ai_impact_analytics_dashboard` feature flag

What does this MR do and why?

Makes AI impact analytics dashboard generally available by removing ai_impact_analytics_dashboard feature flag. A new condition was added to hide the dashboard when Clickhouse has not been configured.

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.

Screenshots or screen recordings

How to set up and validate locally

1. Enable clickhouse locally (docs)
2. Visit the analytics dashboards page: (ex. http://gdk.test:3000/groups/flightjs/-/analytics/dashboards)
3. Validate that the AI impact dashboard appears in the dashboard list
4. Modify config/click_house.yml and rename the development.main entry to something else to disable the clickhouse integration:

development:
  not_main:
    database: gitlab_clickhouse_development
    url: 'http://localhost:8123'

5. gdk restart and reload the analytics dashboard page. The AI impact dashboard should no longer appear in the dashboard list
6. Make sure to revert your changes to config/click_house.yml when you're done testing!

Related to #451873