[CI] `kubectl exec` command uses deprecated format

Summary

Example: https://gitlab.com/gitlab-org/cloud-native/gitlab-operator/-/jobs/1855194872#L150

$ echo "GITLAB_VERSION=$(kubectl -n $TESTS_NAMESPACE exec -ti $(kubectl -n $TESTS_NAMESPACE get pods | grep sidekiq | awk '{print $1}') cat /srv/gitlab/VERSION)" >> review.env
kubectl exec [POD] [COMMAND] is DEPRECATED and will be removed in a future version. Use kubectl exec [POD] -- [COMMAND] instead.
Defaulted container "sidekiq" out of: sidekiq, certificates (init), configure (init), dependencies (init)
Unable to use a TTY - input is not a terminal or the right kind of file

Acceptance criteria

• Add -- before the [COMMAND]
• Specify -c sidekiq
• Remove -t