Update docs for dropping legacy in-cluster PostgreSQL installation

doc/topics/autodevops/upgrading_auto_deploy_dependencies.md

@@ -73,7 +73,9 @@ please proceed with the following upgrade guide. Otherwise, you can skip this pr
 
 #### Kubernetes 1.16+
 
-The v2 auto-deploy-image also drops support for Kubernetes 1.15 and lower.
+The v2 auto-deploy-image drops support for Kubernetes 1.15 and lower. If you need to upgrade your
+Kubernetes cluster, follow your cloud provider's instructions. Here's
+[an example on GKE](https://cloud.google.com/kubernetes-engine/docs/how-to/upgrading-a-cluster).
 
 #### Helm 3
 
@@ -114,6 +116,12 @@ If your Auto DevOps project has an active environment that was deployed with the
    `kubectl apply -f $backup`.
 1. Remove the `MIGRATE_HELM_2TO3` variable.
 
+#### In-Cluster PostgreSQL Channel 2
+
+The v2 auto-deploy-image drops support for [legacy in-cluster PostgreSQL](upgrading_postgresql.md).
+If your Kubernetes cluster still depends on it, [upgrade and migrate your data](upgrading_postgresql.md)
+with the [v1 auto-deploy-image](#use-a-specific-version-of-auto-deploy-dependencies).
+
 #### Traffic routing change for canary deployments and incremental rollouts
 
 > [Introduced](https://gitlab.com/gitlab-org/cluster-integration/auto-deploy-image/-/merge_requests/109) in GitLab 13.4.