Use a different advanced config in test

[Why]
After PR rabbitmq/rabbitmq-server#14359
RabbitMQ verifies that an authN/authZ plugin is enabled, if set in the
configuration. We relied on the old behaviour that simply accepted the
configuration. We have to change our test because the configuration is
not valid anymore since 4.2.0

Author: Zerpet

system_tests/system_test.go

@@ -223,7 +223,7 @@ cluster_keepalive_interval = 10000`
 			By("updating the advanced.config file when advancedConfig are modified", func() {
 				Expect(updateRabbitmqCluster(ctx, rmqClusterClient, cluster.Name, cluster.Namespace, func(cluster *rabbitmqv1beta1.RabbitmqCluster) {
 					cluster.Spec.Rabbitmq.AdvancedConfig = `[
-  {rabbit, [{auth_backends, [rabbit_auth_backend_ldap]}]}
+  {kernel, [{net_ticktime, 61}]}
 ].`
 				})).To(Succeed())
 
@@ -237,7 +237,7 @@ cluster_keepalive_interval = 10000`
 					"/etc/rabbitmq/advanced.config",
 				)
 				Expect(err).NotTo(HaveOccurred())
-				Expect(string(output)).Should(ContainSubstring("[\n  {rabbit, [{auth_backends, [rabbit_auth_backend_ldap]}]}\n]."))
+				Expect(string(output)).Should(ContainSubstring("[\n  {kernel, [{net_ticktime, 61}]}\n]."))
 			})
 
 			By("updating the rabbitmq-env.conf file when additionalConfig are modified", func() {