Migrate VGS from OpenShift private API to Kubernetes public API v1

[ELENAGER]

Summary

• Replace the private groupsnapshot.storage.openshift.io VGS client with the public groupsnapshot.storage.k8s.io/v1 API (external-snapshotter client v8.6.0).
• On upgrade, when both public and private VGS CRDs are present, RGS reconcile deletes Ramen-owned private VGS so they are not left as orphans.
• Bump the drenv external_snapshotter addon to release-8.6 so test clusters match the public VGS CRDs/controller.

Test plan

• make lint / unit tests for affected packages (util, cephfscg, controllers)
• drenv with external_snapshotter addon: confirm VGS CRDs are groupsnapshot.storage.k8s.io/v1 and CephFS CG sync works
• Upgrade-style check: with both public and private VGS CRDs installed and a leftover private VGS labeled created-by-ramen + rgs=<name>, confirm RGS reconcile deletes the private VGS
• Confirm new VGS objects are created only with the public API

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