chore: support only latest, tested versions of Kubernetes

[jackfrancis]

Reason for Change:

This PR removes one version from each minor release from the supported versions list for Azure public cloud. In practice this means the following:

• aks-engine will only support patch version (the most recent to be validated) of Kubernetes per minor release; for example, for 1.17, the most recent, known-working version will be supported, and no other 1.17 patch releases will be supported
• because we only currently test one patch version per minor release (again, the most recently known-working one), this has the effect of improving the test fidelity across Kubernetes versions

This PR makes no changes to the Azure Stack Hub versions configuration, because in practice Azure Stack Hub already only supports one patch version per minor release.

Issue Fixed:

Credit Where Due:

Does this change contain code from or inspired by another project?

• No
• Yes

If "Yes," did you notify that project's maintainers and provide attribution?

• No
• Yes

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[codecov]

Codecov Report

Merging #4265 (57c3e60) into master (3497425) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4265      +/-   ##
==========================================
- Coverage   72.07%   72.06%   -0.01%     
==========================================
  Files         141      141              
  Lines       21686    21686              
==========================================
- Hits        15630    15628       -2     
- Misses       5106     5107       +1     
- Partials      950      951       +1     

Impacted Files	Coverage Δ
pkg/api/common/versions.go	96.37% <ø> (ø)
cmd/get_versions.go	89.47% <0.00%> (-5.27%)	⬇️

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[jadarsie]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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