feat: add support for Kubernetes v1.18.19

[mboersma]

Reason for Change:

See https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.18.md#changelog-since-v11818

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 #4410 (c4afc68) into master (60828c7) will increase coverage by 0.00%.
The diff coverage is 71.71%.

@@           Coverage Diff            @@
##           master    #4410    +/-   ##
========================================
  Coverage   72.04%   72.05%            
========================================
  Files         141      141            
  Lines       21631    21764   +133     
========================================
+ Hits        15584    15681    +97     
- Misses       5096     5131    +35     
- Partials      951      952     +1     

Impacted Files	Coverage Δ
cmd/rotate_certs.go	11.03% <0.00%> (ø)
cmd/upgrade.go	35.92% <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/engine/templates_generated.go	43.31% <ø> (ø)
pkg/engine/template_generator.go	66.37% <11.53%> (-2.17%)	⬇️
cmd/get_logs.go	49.57% <30.43%> (-1.79%)	⬇️
pkg/api/addons.go	98.01% <100.00%> (ø)
pkg/api/converterfromapi.go	95.71% <100.00%> (+<0.01%)	⬆️
... and 3 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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