[v12.x] doc: mark modules implementation as stable

We had kept the implementation as experimental to leave room for large changes that might land prior to 16. With the Semver-Major cut-off looming I don't think we'll be seeing any. Let's mark ESM stable and make it clear that folks can use this in prod

PR-URL: https://github.com/nodejs/node/pull/35781 Reviewed-By: Matteo Collina matteo.collina@gmail.com Reviewed-By: Benjamin Gruenbaum benjamingr@gmail.com Reviewed-By: Rich Trott rtrott@gmail.com Reviewed-By: Michaël Zasso targos@protonmail.com Reviewed-By: Franziska Hinkelmann franziska.hinkelmann@gmail.com Reviewed-By: James M Snell jasnell@gmail.com Reviewed-By: Myles Borins myles.borins@gmail.com Reviewed-By: Antoine du Hamel duhamelantoine1995@gmail.com Reviewed-By: Derek Lewis DerekNonGeneric@inf.is