⚠️ BREAKING CHANGE: switch API group to x-k8s.io

[Huang-Wei]

What type of PR is this?

/kind api-change

What this PR does / why we need it:

It's requested to document all usages of "k8s.io" and "kubernetes.io" in k/website. However, scheduler-plugins is not supposed to be part of upstream.

To comply with https://kubernetes.io/docs/reference/labels-annotations-taints/, we should stop using "k8s.io" and "kubernetes.io" in all places of this repo, including:

• API group
• Labels
• Annotations

Which issue(s) this PR fixes:

Fixes #525

Special notes for your reviewer:

Does this PR introduce a user-facing change?

ACTION REQUIRED:
- The CRDs `PodGroup` and `ElasticQuota` are now using a new API group: `scheduling.x-k8s.io`. When you upgrade scheduler-plugins to version v0.25.x, run `kubectl replace -f <crd yamls>` to register the new CRDs.
- The Pods work with `PodGroup` feature need to migrate to use new label: `scheduling.x-k8s.io/pod-group`.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Huang-Wei

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[Huang-Wei]

/assign @denkensk

[denkensk]

/lgtm

Thanks @Huang-Wei