update docker image tag to v2.21.0 in README.md

[Payback159]

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:

Update docker tag to v2.21.0 in README.md

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Update docker tag to v2.21.0 in README.md

[k8s-ci-robot]

Hi @Payback159. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[oomichi]

Thanks for updating.

/ok-to-test
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: oomichi, Payback159

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[yankay]

Thanks @Payback159
/lgtm