🐛 ci: use KUBERNETES_VERSION in kubeadm script

[jackfrancis]

What this PR does / why we need it:

This PR replaces the capz-specific $CI_VERSION env var with the capi-standard $KUBERNETES_VERSION variable. This was introduced here:

#6711

Folks are seeing such side-effects since the above PR merged:

Jul 06 03:40:39 cluster-conformance-98lra3-control-plane-nhfqm cloud-init[611]: [2022-07-06 03:40:15] /usr/local/bin/ci-artifacts.sh: line 103: CI_VERSION: unbound variable

Sorry everyone, thanks @sbueringer for triaging!

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

[sbueringer]

Thx for the quick fix!

/lgtm
/approve

/cherry-pick release-1.2

/cc @apricote @mdbooth

[k8s-infra-cherrypick-robot]

@sbueringer: once the present PR merges, I will cherry-pick it on top of release-1.2 in a new PR and assign it to you.

In response to this:

Thx for the quick fix!

/lgtm
/approve

/cherry-pick release-1.2

/cc @apricote @mdbooth

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sbueringer

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[k8s-infra-cherrypick-robot]

@sbueringer: new pull request created: #6847

In response to this:

Thx for the quick fix!

/lgtm
/approve

/cherry-pick release-1.2

/cc @apricote @mdbooth

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.