add test matrix for k8s versions

[sozercan]

Signed-off-by: Sertac Ozercan sozercan@gmail.com

What this PR does / why we need it:
Adds a matrix of supported Kubernetes versions for testing for manifest (Helm is pinned to latest)
Kind does not support v1.21 yet, it can be added once supported

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):
Fixes #1075

Special notes for your reviewer:

[codecov-commenter]

Codecov Report

Merging #1268 (bbd91d6) into master (fc67ca8) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1268      +/-   ##
==========================================
- Coverage   49.97%   49.93%   -0.05%     
==========================================
  Files          65       65              
  Lines        4522     4522              
==========================================
- Hits         2260     2258       -2     
- Misses       1952     1954       +2     
  Partials      310      310              

Flag	Coverage Δ
unittests	49.93% <ø> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...onstrainttemplate/constrainttemplate_controller.go	56.63% <0.00%> (-0.65%)	⬇️

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[maxsmythe]

LGTM