Automation check-in and rate limit changes on GitLab.com

Introduced new rate limits for GitLab.com: 500 requests/minute for unauthenticated IP traffic, 2000 requests/minute for authenticated API traffic per user, 1000 requests/minute for other authenticated HTTP traffic per user, and 2000 requests/minute for all other IP traffic. Documented handling of 429 status codes and Retry-After headers for automated traffic.