📖 Fix version support doc

[furkatgofurov7]

What this PR does / why we need it:
This PR fixes the version support doc by adding Kubernetes v1.20 and updating the CABPK support matrix according to latest changes for kubeadm v1beta1 types removal.

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 #4320
Backport of #4301

[k8s-ci-robot]

Hi @furkatgofurov7. Thanks for your PR.

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[furkatgofurov7]

/assign @fabriziopandini

/hold until #4292 is merged.

[randomvariable]

/ok-to-test

[fabriziopandini]

/approve
Let's wait for #4292 + release date before unholding

[fabriziopandini]

/approve cancel
/lgtm
(as per contributors guide)

[vincepri]

/approve
/hold cancel

[vincepri]

/milestone v0.3.x

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fabriziopandini, vincepri

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