- Added a flag to kubectl version to detect whether a client/server version mismatch was outside the officially supported range. ([#127365](https://github.com/kubernetes/kubernetes/pull/127365), [@omerap12](https://github.com/omerap12))

* Upgrade to etcd client 3.3.17 to fix bug where etcd client does not parse IPv6 addresses correctly when members are joining, and to fix bug where failover on multi-member etcd cluster fails certificate check on DNS mismatch ([#83801](https://github.com/kubernetes/kubernetes/pull/83801), [@jpbetz](https://github.com/jpbetz))

 - Kubeadm: remove the automatic detection and matching of cgroup drivers for Docker. For new clusters if you have not configured the cgroup driver explicitly you might get a failure in the `kubelet` on driver mismatch (kubeadm clusters should be using the `systemd` driver). Also remove the `IsDockerSystemdCheck` preflight check (warning) that checks if the Docker cgroup driver is set to `systemd`. Ideally such detection / coordination should be on the side of...

* Upgrade to etcd client 3.3.17 to fix bug where etcd client does not parse IPv6 addresses correctly when members are joining, and to fix bug where failover on multi-member etcd cluster fails certificate check on DNS mismatch ([#83968](https://github.com/kubernetes/kubernetes/pull/83968), [@jpbetz](https://github.com/jpbetz))
* Update to use go1.12.12 ([#84064](https://github.com/kubernetes/kubernetes/pull/84064), [@cblecker](https://github.com/cblecker))

  If the `DeclarativeValidation` feature gate is enabled, mismatches with existing validation are reported via metrics.

- Resolves non-deterministic behavior of the garbage collection controller when ownerReferences with incorrect data are encountered. Events with a reason of `OwnerRefInvalidNamespace` are recorded when namespace mismatches between child and owner objects are detected. The [kubectl-check-ownerreferences](https://github.com/kubernetes-sigs/kubectl-check-ownerreferences) tool can be run prior to upgrading to locate existing objects with invalid ownerReferences.

- Added a new Priority and Fairness metric `apiserver_flowcontrol_request_dispatch_no_accommodation_total` to track the number of times a request dispatch attempt results in a no-accommodation status due to lack of available seats. ([#106629](https://github.com/kubernetes/kubernetes/pull/106629), [@tkashem](https://github.com/tkashem))