Fix detecting default annotation on VolumeGroupSnapshotClass

[nixpanic]

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:

The annotation to use the selected VolumeGroupSnapshotClass by default is expected to be groupsnapshot.storage.kubernetes.io/is-default-class=true, but the check was done on snapshot.storage.kubernetes.io/is-default-class instead.

Which issue(s) this PR fixes:

Fixes #968

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Adding the annotation `groupsnapshot.storage.kubernetes.io/is-default-class=true` on a VolumeGroupSnapshotClass will cause that VolumeGroupSnapshotClass to be used by default.

[nixpanic]

/cc RaunakShah
/assign xing-yang

[k8s-ci-robot]

@nixpanic: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-kubernetes-csi-external-snapshotter-1-29-on-kubernetes-1-29	aa837d1	link	false	/test pull-kubernetes-csi-external-snapshotter-1-29-on-kubernetes-1-29

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[nixpanic]

wow, size/L is rather unexpected. The actual change is 17 lines, the added unit-test blows things up a little

[xing-yang]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nixpanic, xing-yang

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