Bump kubectl in Auto DevOps to latest 1.11.6

Use the latest major version that will support the current clusters on
GKE which is v1.10

changelogs/unreleased/auto-devops-kubectl-1-11-6.yml

+---
+title: Bump kubectl in Auto DevOps to 1.11.6
+merge_request: 24176
+author:
+type: other

lib/gitlab/ci/templates/Auto-DevOps.gitlab-ci.yml

@@ -49,7 +49,7 @@ variables:
   POSTGRES_ENABLED: "true"
   POSTGRES_DB: $CI_ENVIRONMENT_SLUG
 
-  KUBERNETES_VERSION: 1.10.9
+  KUBERNETES_VERSION: 1.11.6
   HELM_VERSION: 2.11.0
 
   DOCKER_DRIVER: overlay2