Having appropriate QueueingHintFn contributes to reducing useless retries and thus improves the overall scheduler's performance.
  
  **How can I migrate?**
  
  For backward compatibility, nil `QueueingHintFn` is treated as always returning QueueAfterBackoff. 
  So, if you want to just keep the existing behavior, you can register `ClusterEventWithHint` with no `QueueingHintFn` in it. 

* For Deployment, ReplicaSet, and DaemonSet, selectors are now immutable when updating via the new `apps/v1beta2` API. For backward compatibility, selectors can still be changed when updating via `apps/v1beta1` or `extensions/v1beta1`. ([#50719](https://github.com/kubernetes/kubernetes/pull/50719), [@crimsonfaith91](https://github.com/crimsonfaith91))

    * Add metrics `*_admission_latencies_milliseconds` and `*_admission_latencies_milliseconds_summary` for backward compatible, but will be removed in a future release.

* Update gogo/protobuf to serialize backward, as to get better performance on deep objects. ([#77355](https://github.com/kubernetes/kubernetes/pull/77355), [@apelisse](https://github.com/apelisse))

- Dropped the additional rule requirement (cronjobs/finalizers) for roles
  using kubectl create cronjobs to ensure backward compatibility. ([#124883](https://github.com/kubernetes/kubernetes/pull/124883), [@ardaguclu](https://github.com/ardaguclu))

`$enable-hover-media-query` are deprecated.\n//\n// Originally added during our alphas and maintained during betas, this mixin was\n// designed to prevent `:hover` stickiness on iOS-an issue where hover styles\n// would persist after initial touch.\n//\n// For backward compatibility, we've kept these mixins and updated them to\n// always return their regular pseudo-classes instead of a shimmed media query.\n//\n// Issue: https://github.com/twbs/bootstrap/issues/25195\n\n@mixin hover() {\n  &:hover { @content; }\n}\n\n@mixin...

`$enable-hover-media-query` are deprecated.\n//\n// Originally added during our alphas and maintained during betas, this mixin was\n// designed to prevent `:hover` stickiness on iOS-an issue where hover styles\n// would persist after initial touch.\n//\n// For backward compatibility, we've kept these mixins and updated them to\n// always return their regular pseudo-classes instead of a shimmed media query.\n//\n// Issue: https://github.com/twbs/bootstrap/issues/25195\n\n@mixin hover() {\n  &:hover { @content; }\n}\n\n@mixin...