Chore: Align cluster service token docs with code

Internally we set the service account name to `gitlab`
(see https://gitlab.com/gitlab-org/gitlab/-/blob/f5f4e40cbd90b4479b5307eebf1715a92e2944aa/app/services/clusters/kubernetes.rb#L5-8).
We should align the docs to the naming scheme used internally.

doc/user/project/clusters/add_remove_clusters.md

@@ -206,7 +206,7 @@ To add a Kubernetes cluster to your project, group, or instance:
          apiVersion: v1
          kind: ServiceAccount
          metadata:
-           name: gitlab-admin
+           name: gitlab
            namespace: kube-system
          ---
          apiVersion: rbac.authorization.k8s.io/v1beta1
@@ -219,7 +219,7 @@ To add a Kubernetes cluster to your project, group, or instance:
            name: cluster-admin
          subjects:
            - kind: ServiceAccount
-             name: gitlab-admin
+             name: gitlab
              namespace: kube-system
          ```
 
@@ -245,23 +245,23 @@ To add a Kubernetes cluster to your project, group, or instance:
          Output:
 
          ```shell
-         serviceaccount "gitlab-admin" created
+         serviceaccount "gitlab" created
          clusterrolebinding "gitlab-admin" created
          ```
 
-      1. Retrieve the token for the `gitlab-admin` service account:
+      1. Retrieve the token for the `gitlab` service account:
 
          ```shell
-         kubectl -n kube-system describe secret $(kubectl -n kube-system get secret | grep gitlab-admin | awk '{print $1}')
+         kubectl -n kube-system describe secret $(kubectl -n kube-system get secret | grep gitlab | awk '{print $1}')
          ```
 
          Copy the `<authentication_token>` value from the output:
 
          ```yaml
-         Name:         gitlab-admin-token-b5zv4
+         Name:         gitlab-token-b5zv4
          Namespace:    kube-system
          Labels:       <none>
-         Annotations:  kubernetes.io/service-account.name=gitlab-admin
+         Annotations:  kubernetes.io/service-account.name=gitlab
                       kubernetes.io/service-account.uid=bcfe66ac-39be-11e8-97e8-026dce96b6e8
 
          Type:  kubernetes.io/service-account-token