Add product badge for the Agent page

Since the Agent is not available to everyone, add badging so the
release post item won't get flagged.

doc/user/clusters/agent/index.md

@@ -4,7 +4,9 @@ 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/#designated-technical-writers
 ---
 
-# GitLab Kubernetes Agent
+# GitLab Kubernetes Agent **(PREMIUM ONLY)**
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/223061) in [GitLab Premium](https://about.gitlab.com/pricing/) 13.4.
 
 ## Goals