Need to add SourceVolumeMode in restore when VSC doesn't have that value

[blackpiglet]

What steps did you take and what happened:

This issue is related to #7433.
After #7433's related PR, the backup and restore with the external-snapshotter version no older than v0.7.0.
If the CSI-related backup is created with the external-snapshotter version older than v0.7.0, and restoring the backup into the k8s environment version no older than v0.7.0, then the restore should still fail with VolumeSnapshotContent not valid.

What did you expect to happen:

The Velero CSI plugin should modify the VolumeSnapshotContetent in RIA.
If the VolumeSnapshotContent's Spec.SourceVolumeMode is nil, the CSI plugin should set the value to Filesystem.

The following information will help us better understand what's going on:

If you are using velero v1.7.0+:
Please use velero debug --backup <backupname> --restore <restorename> to generate the support bundle, and attach to this issue, more options please refer to velero debug --help

If you are using earlier versions:
Please provide the output of the following commands (Pasting long output into a GitHub gist or other pastebin is fine.)

• kubectl logs deployment/velero -n velero
• velero backup describe <backupname> or kubectl get backup/<backupname> -n velero -o yaml
• velero backup logs <backupname>
• velero restore describe <restorename> or kubectl get restore/<restorename> -n velero -o yaml
• velero restore logs <restorename>

Anything else you would like to add:

Environment:

• Velero version (use velero version):
• Velero features (use velero client config get features):
• Kubernetes version (use kubectl version):
• Kubernetes installer & version:
• Cloud provider or hardware configuration:
• OS (e.g. from /etc/os-release):

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[blackpiglet]

I thought the SourceVolumeMode was required after the external-snapshotter version was upgraded to v7.0.1. #7433

After checking the external-snapshotter code, I found there was no need to add the SourceVolumeMode in the VolumeSnapshotContent.

The external-snapshotter webhook never checks the SourceVolumeMode's value for creating the VolumeSnapshotContent.

https://github.com/kubernetes-csi/external-snapshotter/blob/fcf78d3d6964632ed7f8b85aa045d667b1da47d4/pkg/validation-webhook/snapshot.go#L142-L163

For the VolumeSnapshotContent's update case, the webhook can also accept the empty SourceVolumeMode.

https://github.com/kubernetes-csi/external-snapshotter/blob/fcf78d3d6964632ed7f8b85aa045d667b1da47d4/pkg/validation-webhook/snapshot.go#L248-L256

[danfengliu]

Nightly test of migration from v1.13 to 1.14 will cover this case.