Automatic merge from submit-queue (batch tested with PRs 66011, 66111, 66106, 66039, 65745). 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>.

Disable initializers by default

The API group driving this field is not enabled by default, and it does not appear that these will progress beyond the alpha phase.

Remove the admission plugin from default configurations.

/sig api-machinery
/cc @smarterclayton

```release-note
The alpha `Initializers` admission plugin is no longer enabled by default. This matches the off-by-default behavior of the alpha API which drives initializer behavior.
```