test(integration): expose KUBE_VERSION

[mflendrich]

Makes it possible to specify the version of Kubernetes running KIC's integration test.

The choice is limited to available tags of the docker/io/kindest/node docker registry (which doesn't cover all releases at this moment). Asked kind maintainers (kubernetes-sigs/kind#197 (comment)) to increase coverage (specifically: start providing images for pre-release k8s builds as well)

[codecov]

Codecov Report

Merging #1022 (60c67cb) into main (91585dd) will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1022      +/-   ##
==========================================
- Coverage   49.65%   49.56%   -0.10%     
==========================================
  Files          32       32              
  Lines        3198     3198              
==========================================
- Hits         1588     1585       -3     
- Misses       1481     1483       +2     
- Partials      129      130       +1     

Impacted Files	Coverage Δ
internal/ingress/controller/parser/parser.go	83.92% <0.00%> (-1.34%)	⬇️

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[rainest]

IDK what the heck codecov's problem is, but LGTM

[shaneutt]

LGTM

Codecov for some reason seems to have only just fully evaluated parser.go as an effect of this test, which caused it to trigger some new differences with it's base:

https://app.codecov.io/gh/Kong/kubernetes-ingress-controller/compare/1022/changes

I feel like this could have been a bug in previous code coverage not evaluating this on Codecov's end which may be covered up once this merges.

Seems reasonably innocuous but if we wanted to I suppose we could report the bug to codecov and see what they say.