refactor: configure AI Gateway URL in Duo setup script

Relates to issue #543820

This MR was created by Duo in Session 1108437

Changes

• Update GitLab Duo setup script to configure AI Gateway URL via Ai::Setting.current.ai_gateway_url
• Remove deprecated AI_GATEWAY_URL environment variable references
• Centralize AI Gateway URL configuration in database instead of environment variables
• Update documentation to reflect the new configuration approach