Next-generation GitLab container registry goes GA

The next-generation GitLab container registry is now generally available (off by default) for self-managed deployments starting with GitLab 17.3. This release includes stable and reliable registry API, metadata database, and zero-downtime garbage collection. It also introduces opt-in database schema migrations for Charts installs during upgrades. Non-goals for GA include automatic provisioning of the registry database, automatic schema migrations for omnibus installs, automatic import of object storage data, and Geo support. The legacy registry will continue to receive security updates but will stop being supported after GitLab 19.0. Future roadmap items include protected repositories, immutable tags, improved Helm chart management, and enhanced support for signing and attestations.