v7.0.1 API is not backwards compatible

[Lyndon-Li]

What happened:
In v7.0.1 since prevent-volume-mode-conversion is enabled by default, the created volumesnapshotcotent sets its spec.SourceVolumeMode by inheriting value from the source PVC, so it must not be nil. On the other hand, in v7.0.1, there is a change #1005 where the webhook fails the validation of updating volumesnapshotcotent if oldSnapcontent.Spec.SourceVolumeMode != nil && newSnapcontent.Spec.SourceVolumeMode == nil.

However, it is possible that for a client that is using an older version of external-snapshotter API, spec.SourceVolumeMode field does not exist in volumesnapshotcotent's spec. If the Update method is called through this old client, it falls into oldSnapcontent.Spec.SourceVolumeMode != nil && newSnapcontent.Spec.SourceVolumeMode == nil, as a result, the Update will fail.

What you expected to happen:
Update method should succeed even spec.SourceVolumeMode is not defined

How to reproduce it:
See Velero issue vmware-tanzu/velero#7433

Anything else we need to know?:

Environment:

• Driver version:
• Kubernetes version (use kubectl version):
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others: