chore: update cluster-autoscaler to 1.20.0

[jackfrancis]

Reason for Change:

This PR updates the cluster-autoscaler addon to 1.20.0. See:

https://github.com/kubernetes/autoscaler/releases/tag/cluster-autoscaler-1.20.0

As this tested out O.K. with all currently supported versions of Kubernetes (1.17+), we can remove the heterogeneous set of supported cluster-autoscaler versions and pin to 1.20.0 for all versions of k8s.

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:

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[jackfrancis]

cc @mboersma

[codecov]

Codecov Report

Merging #4264 (31a6fab) into master (1c12289) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4264   +/-   ##
=======================================
  Coverage   72.07%   72.07%           
=======================================
  Files         141      141           
  Lines       21686    21686           
=======================================
  Hits        15630    15630           
  Misses       5106     5106           
  Partials      950      950           

Impacted Files	Coverage Δ
pkg/api/k8s_versions.go	100.00% <ø> (ø)

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[mboersma]

pin to 1.20.0 for all versions of k8s.

I thought the idea was to keep cluster-autoscaler in sync with the Kubernetes minor version. That is, CA 1.20.x goes with Kubernetes 1.20.x (and 1.21.x until there's a specific release for it), CA 1.19.x goes with k8s 1.19.x, etc.

Also I see the Azure Pipeline for CA completed except for the publish step--I'll nudge upstream publishing to approve that.

[jackfrancis]

@mboersma As far as our functional cluster-autoscaler tests go, 1.20 works w/ 1.17+, so that's good enough for me. I think until we can prove something doesn't work we shouldn't take on the additional test burden of validating version-to-version CA-k8s affinity. Having a single CA version simplifies our test matrix, and VHD maintenance.

cc @craiglpeters @devigned for any additional thoughts that may not agree with my conclusions above

[mboersma]

I like the simplification of using one cluster-autoscaler--like AKS Engine does for core-dns--the tradeoff is we don't benefit from this upstream testing:

The below combinations have been tested on GCP. We don't do cross version testing or compatibility testing in other environments.

But I guess AKS Engine is doing cross-version testing, in the Azure environment, so I'm ok with that.

[devigned]

Late to the party. Sorry, this notification slipped past.

+1 to Matt's and Jack's comments and to our test matrix. Since we have cross version signal 1.20, we've done pretty well without taking on too much more test burden.