feat: add support for Kubernetes v1.21.1

[mboersma]

Reason for Change:

See https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.20.md#changelog-since-v1210

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

Merging #4407 (355a834) into master (60828c7) will increase coverage by 0.06%.
The diff coverage is 82.64%.

@@            Coverage Diff             @@
##           master    #4407      +/-   ##
==========================================
+ Coverage   72.04%   72.11%   +0.06%     
==========================================
  Files         141      141              
  Lines       21631    21740     +109     
==========================================
+ Hits        15584    15677      +93     
- Misses       5096     5111      +15     
- Partials      951      952       +1     

Impacted Files	Coverage Δ
cmd/rotate_certs.go	11.03% <0.00%> (ø)
pkg/api/common/versions.go	96.37% <ø> (ø)
pkg/api/types.go	92.85% <ø> (ø)
pkg/api/vlabs/types.go	73.04% <ø> (ø)
pkg/api/vlabs/validate.go	81.83% <ø> (ø)
pkg/engine/templates_generated.go	43.31% <ø> (ø)
cmd/get_logs.go	49.57% <23.80%> (-1.79%)	⬇️
pkg/engine/template_generator.go	68.37% <50.00%> (-0.17%)	⬇️
pkg/api/addons.go	98.01% <100.00%> (ø)
pkg/api/converterfromapi.go	95.71% <100.00%> (+<0.01%)	⬆️
... and 2 more

---

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[jackfrancis]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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