feat: add Azure Stack support for k8s 1.18.18, 1.19.10, and 1.20.6

[JesusAlvarezTorres]

Reason for Change:

Adding support for K8s versions 1.18.18, 1.19.10, and 1.20.6 on Azure Stack.

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:

[jadarsie]

@jackfrancis I think this is ok to merge, the check failure seems unrelated to the changes.

[jackfrancis]

/azp run pr-e2e

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jackfrancis]

/lgtm

pending UT

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis, jadarsie, JesusAlvarezTorres

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 [jackfrancis,jadarsie]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[codecov]

Codecov Report

Merging #4396 (2485199) into master (60828c7) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4396   +/-   ##
=======================================
  Coverage   72.04%   72.04%           
=======================================
  Files         141      141           
  Lines       21631    21631           
=======================================
  Hits        15584    15584           
  Misses       5096     5096           
  Partials      951      951           

Impacted Files	Coverage Δ
pkg/api/common/versions.go	96.37% <ø> (ø)
pkg/engine/templates_generated.go	43.31% <ø> (ø)
pkg/api/addons.go	98.01% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1747dcb...2485199. Read the comment docs.