Automatic merge from submit-queue (batch tested with PRs 59353, 59905, 53833). 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>.

Graduate kubeletconfig API group to beta

Regarding https://github.com/kubernetes/features/issues/281, this PR moves the kubeletconfig API group to beta. 

After #53088, the KubeletConfiguration type should not contain any deprecated or experimental fields, and we should not have to remove any more fields from the type before graduating it to beta. 

We need the community to double check for two things, however:
1. Are there any fields currently in the KubeletConfiguration type that you were going to mark deprecated this quarter, but haven't yet?
2. Are there any fields currently in the KubeletConfiguration type that are experimental or alpha, but were not explicitly denoted as such?

Please comment on this PR if you can answer "yes" to either of those two questions. Please cc anyone with a stake in the kubeletconfig API, so we get as much coverage as possible.

/cc @thockin @dchen1107 @Random-Liu @yujuhong @dashpole @tallclair @vishh @abw @freehan @dnardo @bowei @MrHohn @luxas @liggitt @ncdc @derekwaynecarr @mikedanese 

@kubernetes/sig-network-pr-reviews, @kubernetes/sig-node-pr-reviews 

```release-note
action required: The `kubeletconfig` API group has graduated from alpha to beta, and the name has changed to `kubelet.config.k8s.io`. Please use `kubelet.config.k8s.io/v1beta1`, as `kubeletconfig/v1alpha1` is no longer available. 
```

**TODO:**
- [x] Move experimental/non-gated-alpha/soon-to-be-deprecated fields to `KubeletFlags`
  - [x] #53088
  - [x] #54154
  - [x] #54160
  - [x] #55562
  - [x] #55983
  - [x] #57851
- [x] Lift embedded structure out of strings
  - [x] #53025
  - [x] #54643
  - [x] #54823
  - [x] #55254
- [x] Resolve relative paths against the location config files are loaded from
  - [x] #55648 
- [x] Rename to `kubelet.config.k8s.io`
- [x] Comments
  - [x] Make sure existing comments at least read sensibly.
  - [x] Note default values in comments on the versioned struct.
  - [x] Remove any reference to default values in comments on the internal struct.
- [x] Most fields should be `+optional` and `omitempty`. Add where necessary. ~Where omitted, explicitly comment.~ Edit: We should not distinguish between nil and empty, see below items.
- [x] Ensure defaults are specified via `pkg/kubelet/apis/kubelet.config.k8s.io/v1beta1/defaults.go`, not `cmd/kubelet/app/options/options.go`.
  - [x] #57770
- [x] Ensure kubeadm does not persist v1alpha1 KubeletConfiguration objects (or feature-gates this functionality)
- [x] Don't make a distinction between empty and nil, because of #43203.
  - [x] #59515
  - [x] #59681
- [x] Take the opportunity to fix insecure Kubelet defaults @tallclair 
  - [x] #59666
- [x] Remove CAdvisorPort from KubeletConfiguration wrt #56523.
  - [x] #59580
- [x] Hide `ConfigTrialDuration` until we're more sure what to do with it.
   - [x] #59628
- [x] Fix `// default: x` comments after rebasing on recent changes.