Added serverless deprecation

data/deprecations/serverless.yml

+- name: "GitLab Serverless"
+  announcement_milestone: "14.3"
+  removal_milestone: "15.0"
+  body: |
+    [GitLab Serverless](https://docs.gitlab.com/ee/user/project/clusters/serverless/) is a feature set to support Knative-based serverless development with automatic deployments and monitoring.
+
+    We decided to remove the GitLab Serverless features as they never really resonated with our users. Besides, given the continuous development of Kubernetes and Knative, our current implementations do not even work with recent versions.
+  stage: Configure
+  tiers: [Core, Premium, Ultimate]
+  issue_url: "https://gitlab.com/groups/gitlab-org/configure/-/epics/6"
+  documentation_url: "https://docs.gitlab.com/ee/user/project/clusters/serverless/"
+

doc/user/project/clusters/serverless/index.md

@@ -4,9 +4,10 @@ group: Configure
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
 ---
 
-# Serverless **(FREE)**
+# Serverless (DEPRECATED) **(FREE)**
 
-> Introduced in GitLab 11.5.
+> - Introduced in GitLab 11.5.
+> - [Deprecated](https://gitlab.com/groups/gitlab-org/configure/-/epics/6) in GitLab 14.3.
 
 WARNING:
 Serverless is currently in [alpha](https://about.gitlab.com/handbook/product/gitlab-the-product/#alpha).