doc: move ERR_WORKER_UNSPPORTED_EXTENSION to legacy

PR-URL: https://github.com/nodejs/node/pull/39788
Fixes: https://github.com/nodejs/node/issues/39124


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Bradley Farias <bradley.meck@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>