feat: add support for Kubernetes 1.21.2

[mboersma]

Reason for Change:

See https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.21.md#changelog-since-v1211

Issue Fixed:

Credit Where Due:

Does this change contain code from or inspired by another project?

• No
• Yes

Requirements:

• Kubernetes artifacts built and pushed by Azure Pipelines
• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@d0998fa). Click here to learn what that means.
The diff coverage is 38.28%.

❗ Current head 8953a5c differs from pull request most recent head 73635db. Consider uploading reports for the commit 73635db to get more accurate results

@@            Coverage Diff            @@
##             master    #4471   +/-   ##
=========================================
  Coverage          ?   72.05%           
=========================================
  Files             ?      141           
  Lines             ?    21765           
  Branches          ?        0           
=========================================
  Hits              ?    15682           
  Misses            ?     5131           
  Partials          ?      952           

Impacted Files	Coverage Δ
cmd/rotatecerts/arm.go	0.00% <0.00%> (ø)
cmd/rotate_certs.go	11.03% <11.03%> (ø)
cmd/scale.go	12.56% <12.37%> (ø)
cmd/addpool.go	17.97% <17.97%> (ø)
cmd/update.go	23.07% <23.07%> (ø)
cmd/upgrade.go	35.92% <25.00%> (ø)
cmd/get_logs.go	49.57% <49.57%> (ø)
cmd/get_skus.go	50.63% <50.63%> (ø)
cmd/root.go	56.28% <52.63%> (ø)
cmd/generate.go	50.41% <54.54%> (ø)
... and 7 more

---

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[Michael-Sinz]

/lgtm

[mboersma]

/azp run aks-engine-windows-vhd-validate

[azure-pipelines]

No pipelines are associated with this pull request.

[jackfrancis]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis, mboersma, Michael-Sinz

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Needs approval from an approver in each of these files:

• OWNERS [jackfrancis,mboersma]

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