Deprecate Attached Cluster Documentation

doc/user/project/clusters/add_remove_clusters.md

@@ -6,6 +6,15 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 
 # Add a cluster using cluster certificates **(FREE)**
 
+> [Deprecated](https://gitlab.com/groups/gitlab-org/-/epics/6049) in GitLab 14.0.
+
+WARNING:
+Creating a new cluster or adding an existing cluster to GitLab through the certificate-based method
+is deprecated and no longer recommended. Kubernetes cluster, similar to any other
+infrastructure, should be created, updated, and maintained using [Infrastructure as Code](../../infrastructure/index.md).
+GitLab is developing a built-in capability to create clusters with Terraform.
+You can follow along in this [epic](https://gitlab.com/groups/gitlab-org/-/epics/6049). 
+
 GitLab offers integrated cluster creation for the following Kubernetes providers:
 
 - Google Kubernetes Engine (GKE).