gVisor: bump release to 20210921 version

[cristicalin]

What type of PR is this?

/kind feature

What this PR does / why we need it:
This PR updates the gVisor to the latest version available.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[gVisor] Update gVisor to 20210921 release

[k8s-ci-robot]

Hi @cristicalin. Thanks for your PR.

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[floryut]

/ok-to-test

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cristicalin, floryut

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[oomichi]

Thanks for doing this.

/lgtm