promote kep-3673 to GA

[pacoxu]

• One-line PR description: kep 3673 GA

• Issue link: Kubelet limit of Parallel Image Pulls #3673

• Other comments: for GA, we change default behavior to serializeImagePulls: false and maxParallelImagePulls: 2.

[pacoxu]

/cc @ruiwen-zhao
/assign @wojtek-t @SergeyKanzhelev

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/cc @ruiwen-zhao
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[SergeyKanzhelev]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: mrunalp, pacoxu, SergeyKanzhelev
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