Add ISTIO_SYSTEM_NAMESPACE constant

Relates to https://gitlab.com/gitlab-org/gitlab/issues/33559

app/models/clusters/applications/knative.rb

@@ -74,7 +74,7 @@ module Clusters
       end
 
       def ingress_service
-        cluster.kubeclient.get_service('istio-ingressgateway', 'istio-system')
+        cluster.kubeclient.get_service('istio-ingressgateway', Clusters::Kubernetes::ISTIO_SYSTEM_NAMESPACE)
       end
 
       def uninstall_command

app/services/clusters/kubernetes.rb

@@ -12,5 +12,7 @@ module Clusters
     GITLAB_KNATIVE_SERVING_ROLE_BINDING_NAME = 'gitlab-knative-serving-rolebinding'
     GITLAB_CROSSPLANE_DATABASE_ROLE_NAME = 'gitlab-crossplane-database-role'
     GITLAB_CROSSPLANE_DATABASE_ROLE_BINDING_NAME = 'gitlab-crossplane-database-rolebinding'
+    KNATIVE_SERVING_NAMESPACE = 'knative-serving'
+    ISTIO_SYSTEM_NAMESPACE = 'istio-system'
   end
 end