doc: add changelog for mocktimers

Signed-off-by: Erick Wendel <[email protected]>
PR-URL: #55398
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Marco Ippolito <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>
Reviewed-By: Moshe Atlow <[email protected]>
Reviewed-By: Jake Yuesong Li <[email protected]>
Reviewed-By: Chemi Atlow <[email protected]>

Author: ErickWendel

doc/api/test.md

@@ -2224,6 +2224,10 @@ 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