Updated K8s ClusterRoleBinding API version

Updated the ClusterRoleBinding on the gitlab-admin-service-account.yaml
snippet provided while adding a cluster.

V1 of the API has been available since at least 1.16, and v1beta1 is
currently deprecated.
Signed-off-by: Raimund Hook <raimund.hook@exfo.com>

changelogs/unreleased/clusterrolebinding-v1.yml

+---
+title: Updated documented K8s snippet to undeprecated API
+merge_request: 57100
+author: Raimund Hook (@stingrayza)
+type: other

doc/user/project/clusters/add_remove_clusters.md

@@ -231,7 +231,7 @@ To add a Kubernetes cluster to your project, group, or instance:
            name: gitlab
            namespace: kube-system
          ---
-         apiVersion: rbac.authorization.k8s.io/v1beta1
+         apiVersion: rbac.authorization.k8s.io/v1
          kind: ClusterRoleBinding
          metadata:
            name: gitlab-admin