Deployment Protection: Added security controls now available on all plans

Introduces Deployment Protection features: Vercel Authentication for securing preview deployments with Vercel accounts, Shareable Links for external collaborators without login, and Advanced Deployment Protection for enhanced security including header secrets for automation (E2E testing), password protection, and private production deployments. Vercel Authentication replaces SSO Protection for Enterprise. Advanced Deployment Protection is available as an add-on for Pro and included in Enterprise plans.