feat: add support for Kubernetes 1.21.0-alpha.3

[mboersma]

Reason for Change:

See https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.21.md#changelog-since-v1210-alpha2

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 #4258 (b6aa398) into master (6a8a5af) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4258   +/-   ##
=======================================
  Coverage   72.07%   72.07%           
=======================================
  Files         141      141           
  Lines       21676    21676           
=======================================
  Hits        15622    15622           
  Misses       5105     5105           
  Partials      949      949           

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

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[jackfrancis]

Can we remove the orchestrator version allow list from this test config file?

• test/e2e/test_cluster_configs/availabilityset.json

Then I'll run the test matrix against that config to validate the 1.21 Windows + availability set + LB thing is fixed w/ alpha.3 (we have evidence it should be)

Thanks!

[jackfrancis]

nevermind that reversion already landed!

[jackfrancis]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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