Remove volumeGroupSnapshotRef immutable validation rule

[xing-yang]

What type of PR is this?

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/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:
We need to remove the validation rule that ensures volumeGroupSnapshotRef is immutable.
This is because that UID needs to be set by the snapshot controller after volumeGroupSnapshotRef is populated with name and namespace for pre-provisioned VolumeGroupSnapshotContent.

This immutable validation rule was added as part of this PR #1073 to use CEL to replace the validation added in the validation webhook:

	// +kubebuilder:validation:XValidation:rule="self == oldSelf",message="volumeGroupSnapshotRef is immutable"

However, in the original validation webhook, we never had this rule:
https://github.com/kubernetes-csi/external-snapshotter/blob/v7.0.2/pkg/validation-webhook/validation.go#L52
With this rule, the snapshot controller won't be able to set UID for volumeGroupSnapshotRef after it was populated with name and namespace initially. So we need to remove this rule.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Remove the rule that ensures volumeGroupSnapshotRef is immutable. UID needs to be set by the snapshot controller after volumeGroupSnapshotRef is populated with name and namespace for pre-provisioned VolumeGroupSnapshotContent.

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[leonardoce]

/assign

[leonardoce]

I tested the volume group snapshot static and dynamic provisioning, and everything is fine.

[leonardoce]

/lgtm