diagnostics_channel: mark as stable

PR-URL: https://github.com/nodejs/node/pull/45290
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

doc/api/diagnostics_channel.md

@@ -1,8 +1,16 @@
 # Diagnostics Channel
 
+<!-- YAML
+added: v15.1.0
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/45290
+    description: diagnostics_channel is now Stable.
+-->
+
 <!--introduced_in=v15.1.0-->
 
-> Stability: 1 - Experimental
+> Stability: 2 - Stable
 
 <!-- source_link=lib/diagnostics_channel.js -->