Update VolumeGroupSnapshot to v1beta1

[leonardoce]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change /kind bug /kind cleanup /kind design /kind documentation /kind failing-test /kind feature /kind flake

What this PR does / why we need it:

Update the definition of CRDs to move VolumeGroupSnapshot, VolumeGroupSnapshotContent, and VolumeGroupSnapshotClass from v1alpha1 to v1beta1.

Which issue(s) this PR fixes:

Fixes #1119

Special notes for your reviewer:

The updated CRD still allows the API server to serve v1alpha1, allowing the current clients to continue working correctly. We may unset the served flag later, or decide to set it to false now.

This table is a summary of it:

Version	Stored	Served
v1alpha1	false	true
v1beta1	true	true

There's no conversion between different versions, as the structure is consistent.

Does this PR introduce a user-facing change?:

`VolumeGroupSnapshot`, `VolumeGroupSnapshotContent`, and `VolumeGroupSnapshotClass`
are now available in `v1beta1` version.
No structural difference exists between the `v1alpha1` and `v1beta1` API, and the `v1alpha1` API
is still being served.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: leonardoce Once this PR has been reviewed and has the lgtm label, please assign msau42 for approval. For more information see the Kubernetes Code Review Process.

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Needs approval from an approver in each of these files: - **[OWNERS](https://github.com/kubernetes-csi/external-snapshotter/blob/master/OWNERS)** Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[nixpanic]

/cc xing-yang