- Resolves a spurious "Unknown discovery response content-type" error in client-go discovery requests by tolerating extra content-type parameters in API responses ([#117637](https://github.com/kubernetes/kubernetes/pull/117637), [@seans3](https://github.com/seans3)) [SIG API Machinery]

- Fixed a bug that was blocking extensible error handling when serializing API responses error out. Previously, serialization failures always resulted in the status code of the original response being returned. Now, the following behavior occurs: ([#67041](https://github.com/kubernetes/kubernetes/pull/67041), [@tristanburgess](https://github.com/tristanburgess))

 `http_request_size_bytes`
 `http_request_size_bytes_sum`
 `http_request_size_bytes_count`
 `http_requests_total, http_response_size_bytes`
 `http_response_size_bytes_sum`
 `http_response_size_bytes_count`

error if it is not available.  Clients may request fallback to the normal object by adding a non-qualified mime-type to their `Accept` header like `application/json` - the server will then respond with either the alternate representation if it is supported or the fallback mime-type which is the normal object response. ([#59059](https://github.com/kubernetes/kubernetes/pull/59059), [@smarterclayton](https://github.com/smarterclayton))

* kube-apiserver now uses SSH tunnels for webhooks if...

* Correct wording of kubeadm upgrade response for missing ConfigMap. ([#53337](https://github.com/kubernetes/kubernetes/pull/53337), [@jmhardison](https://github.com/jmhardison))

- The `export` query parameter (inconsistently supported by API resources and deprecated in v1.14) is fully removed.  Requests setting this query parameter will now receive a 400 status response. ([#98312](https://github.com/kubernetes/kubernetes/pull/98312), [@deads2k](https://github.com/deads2k)) [SIG API Machinery, Auth and Testing]

- Resolves a spurious "Unknown discovery response content-type" error in client-go discovery requests by tolerating extra content-type parameters in API responses ([#117571](https://github.com/kubernetes/kubernetes/pull/117571), [@seans3](https://github.com/seans3)) [SIG API Machinery]

  The API Server will no longer proxy non-101 responses for upgrade requests. This could break proxied backends (such as an extension API server) that respond to upgrade requests with a non-101 response code. ([#92941](https://github.com/kubernetes/kubernetes/pull/92941), [@tallclair](https://github.com/tallclair)) [SIG API Machinery]
 
## Changes by Kind

## Before Upgrading

* When Response is a `metav1.Status`, it is no longer copied into the audit.Event status. Only the "status", "reason" and "code" fields are set.  For example, when we run `kubectl get pods abc`, the API Server returns a status object:

* Audit logs might impact the API server performance and the latency of large request and response calls. The issue is observed under the following conditions: if `AdvancedAuditing` feature gate is enabled, which is the default case, if audit logging uses the log backend in JSON format, or if the audit policy records large API calls for requests or responses.

For more information, see [#51899](https://github.com/kubernetes/kubernetes/issues/51899).