net: deprecate _setSimultaneousAccepts() undocumented function

This is an undocumented utility function that is of questionable
utility.

Fixes: https://github.com/nodejs/node/issues/18391

PR-URL: https://github.com/nodejs/node/pull/23760


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>