doc: runtimeConfig can be used to enable deprecated APIs

[kalexmills]

Closes #1271.

This PR documents how to enable deprecated APIs via runtimeConfig.

This ticket is a bit old, so I tested that this still works via kind v0.27.0, running images for k8s v1.29.14.

kind: Cluster
apiVersion: kind.x-k8s.io/v1alpha4
nodes:
  - role: control-plane
    image: kindest/node:v1.29.14@sha256:8703bd94ee24e51b778d5556ae310c6c0fa67d761fae6379c8e0bb480e6fea29
  - role: worker
    image: kindest/node:v1.29.14@sha256:8703bd94ee24e51b778d5556ae310c6c0fa67d761fae6379c8e0bb480e6fea29
runtimeConfig:
  "api/alpha": "true"

After creating the cluster I can see the ValidatingAdmissionPolicy is enabled at v1alpha1. In k8s v1.29, the v1alpha1 version of this resource was deprecated in favor of v1beta1 (link).

$  k api-resources | grep admission
mutatingwebhookconfigurations                    admissionregistration.k8s.io/v1         false        MutatingWebhookConfiguration
validatingadmissionpolicies                      admissionregistration.k8s.io/v1alpha1   false        ValidatingAdmissionPolicy
validatingadmissionpolicybindings                admissionregistration.k8s.io/v1alpha1   false        ValidatingAdmissionPolicyBinding
validatingwebhookconfigurations                  admissionregistration.k8s.io/v1         false        ValidatingWebhookConfiguration

Creating a cluster without the runtimeConfig above results in no ValidatingAdmissionPolicies being shown.

$  k api-resources | grep admission
mutatingwebhookconfigurations                  admissionregistration.k8s.io/v1   false        MutatingWebhookConfiguration
validatingwebhookconfigurations                admissionregistration.k8s.io/v1   false        ValidatingWebhookConfiguration
```

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[stmcginnis]

Seems like a nice breadcrumb to include for someone looking for how to do this.

/ok-to-test
/lgtm

[kalexmills]

/assign @BenTheElder

[kalexmills]

/test

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[kalexmills]

Looks to me like all this is waiting for is approval. Sorry for the comment spam.

[BenTheElder]

/lgtm
/approve
thanks!

we may also have to update these docs soon for kubernetes/kubernetes#133697

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