Add removal warning to WAF docs

doc/user/clusters/applications.md

@@ -1268,6 +1268,11 @@ record.
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/21966) in GitLab 12.7.
 
+WARNING:
+The Web Application Firewall is in its end-of-life process. It is [deprecated](https://gitlab.com/gitlab-org/gitlab/-/issues/271276)
+in GitLab 13.6, and planned for [removal](https://gitlab.com/gitlab-org/gitlab/-/issues/271349)
+in GitLab 14.0.
+
 A Web Application Firewall (WAF) examines traffic being sent or received,
 and can block malicious traffic before it reaches your application. The benefits
 of a WAF are:

doc/user/project/clusters/protect/web_application_firewall/index.md

@@ -6,6 +6,11 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 
 # Web Application Firewall
 
+WARNING:
+The Web Application Firewall is in its end-of-life process. It is [deprecated](https://gitlab.com/gitlab-org/gitlab/-/issues/271276)
+in GitLab 13.6, and planned for [removal](https://gitlab.com/gitlab-org/gitlab/-/issues/271349)
+in GitLab 14.0.
+
 A web application firewall (or WAF) filters, monitors, and blocks HTTP traffic to
 and from a web application. By inspecting HTTP traffic, it can prevent attacks
 stemming from web application security flaws. It can be used to detect SQL injection,

doc/user/project/clusters/protect/web_application_firewall/quick_start_guide.md

@@ -6,6 +6,11 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 
 # Getting started with the Web Application Firewall
 
+WARNING:
+The Web Application Firewall is in its end-of-life process. It is [deprecated](https://gitlab.com/gitlab-org/gitlab/-/issues/271276)
+in GitLab 13.6, and planned for [removal](https://gitlab.com/gitlab-org/gitlab/-/issues/271349)
+in GitLab 14.0.
+
 This is a step-by-step guide to help you use the GitLab [Web Application Firewall](index.md) after
 deploying a project hosted on GitLab.com to Google Kubernetes Engine using [Auto DevOps](../../../../../topics/autodevops/index.md).