feat: add support for Kubernetes v1.21.0-rc.0

[mboersma]

Reason for Change:

See https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.21.md#changelog-since-v1210-beta1

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:

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mboersma

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [mboersma]

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[codecov]

Codecov Report

Merging #4348 (3300fc1) into master (7ae3df7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4348   +/-   ##
=======================================
  Coverage   72.07%   72.07%           
=======================================
  Files         141      141           
  Lines       21640    21640           
=======================================
  Hits        15596    15596           
  Misses       5093     5093           
  Partials      951      951           

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

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