Bump KUBERNETES_VERSION to latest 1.10 series

This allows us to use a kubectl binary that should be able to
connect to Kubernetes clusters v1.9 and up.

changelogs/unreleased/53450-wrong-value-for-kubernetes_version-variable.yml

+---
+title: Bump KUBERNETES_VERSION for Auto DevOps to latest 1.10 series
+merge_request: 22757
+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: 1.8.6
+  KUBERNETES_VERSION: 1.10.9
   HELM_VERSION: 2.11.0
 
   DOCKER_DRIVER: overlay2