Old flags used in ensure-kubectl.sh

[lentzi90]

/kind bug

What steps did you take and what happened:
A periodic job failed due to

$ kubectl version --client --short
error: unknown flag: --short 

in ensure-kubectl.sh.

See: https://prow.k8s.io/view/gs/kubernetes-jenkins/logs/periodic-cluster-api-provider-openstack-e2e-full-test-main/1711561098999631872

What did you expect to happen:

The job should not fail.

Anything else you would like to add:

We just need to update the way we extract the version from kubectl version.

[mdbooth]

Possible fix(es) in #1713