Deprecate support for Kubernetes 1.11

data/release_posts/unreleased/kubernetes.yml

+# DEPRECATIONS (optional)
+deprecations:
+  - feature_name: "Kubernetes 1.11 no longer supported"
+    due: GitLab 12.6
+    reporter: ljlane
+    issue_url: 'https://gitlab.com/gitlab-org/charts/gitlab/issues/1583'
+    documentation_link: 'https://docs.gitlab.com/charts/installation/'
+    description: |
+      When deploying GitLab in Kubernetes, the minimum supported version of Kubernetes is now 1.12. Kubernetes 1.11 is [no longer maintained upstream](https://kubernetes.io/docs/setup/release/version-skew-policy/#supported-versions), and has recently been dropped by popular container management services such as GKE and EKS. GitLab uses Kubernetes 1.13 by default.   
+