Migrate from k8s.gcr.io to registry.k8s.io

[shikharcodes]

Which issue(s) this PR fixes:
This PR is a part of https://github.com/kubernetes/k8s.io/issues/4780.

What this PR does, why we need it:
This PR does the following changes:

• modify all occurrences of k8s.gcr.io to registry.k8s.io.

[knative-prow[bot]]

Welcome @shikharcodes! It looks like this is your first PR to knative/test-infra 🎉

[knative-prow[bot]]

Hi @shikharcodes. Thanks for your PR.

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[knative-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: shikharcodes, upodroid

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