net: deprecate and remove `Server.prototype._listen2` legacy alias

[araujogui]

Server.prototype._listen2 is an undocumented alias for the internal setupListenHandle:

// lib/net.js
Server.prototype._listen2 = setupListenHandle;  // legacy alias

The function was renamed from _listen2 to setupListenHandle in 0ea45707a4; the alias was kept for back-compat.

It's undocumented and nothing in core calls it except listenInCluster, which routes through server._listen2(...) only to preserve an undocumented monkeypatch point (per the inline comment).

Proposal:

1. Deprecate the _listen2 alias.
2. After the deprecation cycle, remove it and call setupListenHandle(...) directly in listenInCluster.