- Introduce support for CEL optionals (see [CEL spec proposal 246](https://github.com/google/cel-spec/wiki/proposal-246)).
  This feature will not be fully enabled until a future Kubernetes release (likely to be v1.29), but is added in v1.28 to enable
  safe rollback on downgrade. ([#118339](https://github.com/kubernetes/kubernetes/pull/118339), [@jpbetz](https://github.com/jpbetz)) [SIG API Machinery, Auth, Cloud Provider and Testing]