feat: add support for Kubernetes v1.18.8

[mboersma]

Reason for Change:
See https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.18.md#changelog-since-v1186
and https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.18.md#changelog-since-v1187

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 #3697 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3697   +/-   ##
=======================================
  Coverage   73.16%   73.16%           
=======================================
  Files         147      147           
  Lines       25318    25318           
=======================================
  Hits        18525    18525           
  Misses       5655     5655           
  Partials     1138     1138           

Impacted Files	Coverage Δ
pkg/api/common/versions.go	96.84% <ø> (ø)

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[CecileRobertMichon]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[dennis-benzinger-hybris]

Do you have an ETA of a release with this PR ?

[mboersma]

@dennis-benzinger-hybris we'll do our best to build the new VHDs and do an AKS Engine release this week, assuming all our testing signal looks good. cc: @jackfrancis.