Raise minimum K8s version to 1.12

As part of upgrading to kubectl 1.13
in https://gitlab.com/gitlab-org/gitlab/issues/30495
we also should raise the minimum cluster version, as
https://kubernetes.io/docs/setup/release/version-skew-policy/#kubectl
means we support 1.12, 1.13 and 1.14

See also https://gitlab.com/gitlab-org/charts/gitlab/issues/1389

doc/topics/autodevops/index.md

@@ -99,7 +99,7 @@ To make full use of Auto DevOps, you will need:
 
   To enable deployments, you will need:
 
-  1. A [Kubernetes 1.5+ cluster](../../user/project/clusters/index.md) for the project. The easiest
+  1. A [Kubernetes 1.12+ cluster](../../user/project/clusters/index.md) for the project. The easiest
      way is to add a [new GKE cluster using the GitLab UI](../../user/project/clusters/add_remove_clusters.md#add-new-gke-cluster).
   1. NGINX Ingress. You can deploy it to your Kubernetes cluster by installing
      the [GitLab-managed app for Ingress](../../user/clusters/applications.md#ingress),