Updated K8s ClusterRoleBinding API version

What does this MR do?

Updated the ClusterRoleBinding on the gitlab-admin-service-account.yaml snippet provided while adding a cluster. (in doc/user/project/clusters/add_remove_clusters.md)

v1 of the API has been available since at least 1.16, and v1beta1 is currently deprecated.

Screenshots (strongly suggested)

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