doc: remove "this API is unstable" note for v8 serdes API

As #30234 marked this as stable I think this line
should be removed as well?

Refs: https://github.com/nodejs/node/pull/30234
PR-URL: https://github.com/nodejs/node/pull/30631


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>