feat: add support for Kubernetes v1.18.9

[mboersma]

Reason for Change:
https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.18.md#changelog-since-v1188

Issue Fixed:

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 #3841 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3841   +/-   ##
=======================================
  Coverage   72.83%   72.83%           
=======================================
  Files         149      149           
  Lines       23171    23171           
=======================================
  Hits        16876    16876           
  Misses       5178     5178           
  Partials     1117     1117           

Impacted Files	Coverage Δ
pkg/api/common/versions.go	96.37% <ø> (ø)

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[jackfrancis]

cc @Michael-Sinz

The next (v0.57.0) release of aks-engine won't include 1.18.6 in the VHD, nor support new cluster creates using v1.18.6

[jackfrancis]

/lgtm

pending rebase

[acs-bot]

New changes are detected. LGTM label has been removed.

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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