Automatic merge from submit-queue (batch tested with PRs 51321, 55969, 55039, 56183, 55976). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
Move mutating to run last in the mutating admission plugin chain
ref: kubernetes/features#492
Follow up on #54892. (see https://github.com/kubernetes/kubernetes/pull/54892#discussion_r151333585)
Only the last commit is relevant.
The reasons are:
* Mutating webhooks are dynamic, they can always adjust according to the behavior of compiled-in admission plugins, but not the other-way around.
* We'll document that if user deploys mutating webhooks that over some built-in mutating plugins decision, user needs also to disable the built-in validating plugins, otherwise the cluster might block.
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