Update external-snapshotter client to v6

[RaunakShah]

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:

This PR updates the external snapshotter client version to the latest release cut- https://github.com/kubernetes-csi/external-snapshotter/releases/tag/client%2Fv6.0.0-rc1

This is needed for changes needed in VolumeSnapshot API

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Update external-snapshotter to v6.0.0-rc1

[RaunakShah]

/assign @xing-yang

[ggriffiths]

/lgtm

[xing-yang]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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