v8: mark serdes API as stable

This has been around for a long time, and the underlying V8 API has
become stable as well a while ago.

PR-URL: #30234
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/v8.md

@@ -266,8 +266,6 @@ if (isMainThread) {
 
 ## Serialization API
 
-> Stability: 1 - Experimental
-
 The serialization API provides means of serializing JavaScript values in a way
 that is compatible with the [HTML structured clone algorithm][].
 The format is backward-compatible (i.e. safe to store to disk).