Extensibility. SIG API Machinery added a new class of admission control webhooks (mutating), and brought the admission control webhooks to beta.

### Apps

The core workloads API, which is composed of the DaemonSet, Deployment, ReplicaSet, and StatefulSet kinds, has been promoted to GA stability in the apps/v1 group version. As such, the apps/v1beta2 group version is deprecated, and all new code should use the kinds in the apps/v1 group version.

### Auth

* Fixes an issue with RBAC reconciliation that could cause duplicated subjects in some bootstrapped rolebindings on each restart of the API server. ([#53239](https://github.com/kubernetes/kubernetes/pull/53239), [@enj](https://github.com/enj))

- When scaling down a ReplicaSet, delete doubled up replicas first, where a "doubled up replica" is defined as one that is on the same node as an active replica belonging to a related ReplicaSet.  ReplicaSets are considered "related" if they have a common controller (typically a Deployment). ([#80004](https://github.com/kubernetes/kubernetes/pull/80004), [...

* [fluentd-gcp addon] Update Stackdriver plugin to version 0.6.7 ([#52565](https://github.com/kubernetes/kubernetes/pull/52565), [@crassirostris](https://github.com/crassirostris))
* Fixes an issue with RBAC reconciliation that could cause duplicated subjects in some bootstrapped rolebindings on each restart of the API server. ([#53239](https://github.com/kubernetes/kubernetes/pull/53239), [@enj](https://github.com/enj))

    - [DaemonSet](#daemonset-1)
    - [Deployment](#deployment-2)
    - [Node](#node)
    - [Pod](#pod)
    - [Pod Security Policy](#pod-security-policy)
    - [RBAC](#rbac-2)
    - [ReplicaSet](#replicaset)
    - [Secrets](#secrets)
    - [Service](#service-1)
    - [StatefulSet](#statefulset-1)
    - [Taints and Tolerations](#taints-and-tolerations)
    - [Volumes](#volumes-2)

  // This will be appended to the base nameservers generated from DNSPolicy.
  // Duplicated nameservers will be removed.
  // +optional
  repeated string nameservers = 1;

  // A list of DNS search domains for host-name lookup.
  // This will be appended to the base search paths generated from DNSPolicy.
  // Duplicated search paths will be removed.
  // +optional
  repeated string searches = 2;

  - A server-side apply request that changes an object which has duplicate items in a keyed list, and modifies the duplicated item removes the duplicates and replaces them with the single item contained in the server-side apply request. ([#121575](https://github.com/kubernetes/kubernetes/pull/121575), [@apelisse](https://github.com/apelisse))

* kubectl and client-go now detect duplicated names for user, cluster and context when loading kubeconfig and report this condition as an error.  ([#60464](https://github.com/kubernetes/kubernetes/pull/60464), [@roycaihw](https://github.com/roycaihw))

- The `controller.kubernetes.io/pod-deletion-cost` annotation can be set to offer a hint on the cost of deleting a `Pod` compared to other pods belonging to the same ReplicaSet. Pods with lower deletion cost are deleted first. This is an alpha feature. ([#99163](https://github.com/kubernetes/kubernetes/pull/99163), [@ahg-g](https://github.com/ahg-g))

- Fixed a regression in default configurations, which enabled PodDisruptionConditions by default, 
  that prevented the control plane's pod garbage collector from deleting pods that contained duplicated field keys (env. variables with repeated keys or container ports). ([#121380](https://github.com/kubernetes/kubernetes/pull/121380), [@mimowo](https://github.com/mimowo)) [SIG Apps, Auth, Scheduling and Testing]