doc: add changelog for mocktimers

Signed-off-by: Erick Wendel <erick.workspace@gmail.com>
PR-URL: #55398
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Jake Yuesong Li <jake.yuesong@gmail.com>
Reviewed-By: Chemi Atlow <chemi@atlow.co.il>

doc/api/test.md

@@ -2222,6 +2222,10 @@ set to `true`.
 added:
   - v20.4.0
   - v18.19.0
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/55398
+    description: The Mock Timers is now stable.
 -->
 
 > Stability: 2 - Stable