feat: add support for Kubernetes v1.20.4

[mboersma]

Reason for Change:

See https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.20.md#changelog-since-v1203

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:
This will fail for the time being, as we are still working out a build issue on the Azure Pipeline for k8s.
v1.20.3 was superceded by v1.20.4 due to a conformance test fix.

[codecov]

Codecov Report

Merging #4285 (366cc93) into master (6726888) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4285      +/-   ##
==========================================
+ Coverage   72.02%   72.06%   +0.03%     
==========================================
  Files         141      141              
  Lines       21625    21686      +61     
==========================================
+ Hits        15576    15628      +52     
- Misses       5099     5107       +8     
- Partials      950      951       +1     

Impacted Files	Coverage Δ
pkg/api/common/versions.go	96.37% <ø> (ø)
pkg/engine/templates_generated.go	43.42% <0.00%> (-0.14%)	⬇️
pkg/api/vlabs/validate.go	81.35% <0.00%> (-0.08%)	⬇️
pkg/api/converterfromapi.go	95.70% <0.00%> (+<0.01%)	⬆️
pkg/api/types.go	92.86% <0.00%> (+0.01%)	⬆️
pkg/api/convertertoapi.go	94.02% <0.00%> (+0.01%)	⬆️
pkg/engine/artifacts.go	98.95% <0.00%> (+0.04%)	⬆️
pkg/api/addons.go	98.10% <0.00%> (+0.07%)	⬆️
pkg/api/defaults-custom-cloud-profile.go	85.24% <0.00%> (+0.12%)	⬆️
pkg/engine/template_generator.go	68.34% <0.00%> (+0.14%)	⬆️
... and 1 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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[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [jackfrancis,mboersma]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment