feat: add support for Kubernetes v1.21.0

[mboersma]

Reason for Change:

See https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.21.md#changelog-since-v1200-1

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 #4362 (f816ac6) into master (ac6d832) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4362   +/-   ##
=======================================
  Coverage   72.08%   72.08%           
=======================================
  Files         141      141           
  Lines       21662    21662           
=======================================
  Hits        15616    15616           
  Misses       5094     5094           
  Partials      952      952           

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

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[jackfrancis]

/lgtm

❤️ @mboersma

[acs-bot]

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