Use dl.k8s.io instead of kubernetes-release bucket

[askulkarni2]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Part of umbrella issues to migrate the kubernetes project away from use of GCP project google-containers: kubernetes/k8s.io#1571 and kubernetes/k8s.io#2396.

Updates hack/ensure-kubectl.sh script to use the https://dl.k8s.io/ url to install kubectl from.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

Checklist:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

Release note:

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @askulkarni2. Thanks for your PR.

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[askulkarni2]

/release-note-none

[Ankitasw]

/test pull-cluster-api-provider-aws-e2e-blocking

[Ankitasw]

/lgtm
/approve

Thanks @askulkarni2 🎉

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Ankitasw

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