net: treat ENOTCONN at shutdown as success

While it is not entirely clear why this condition is being
triggered, it does resolve a reported bug.

Fixes: https://github.com/nodejs/node/issues/26315

Backport-PR-URL: https://github.com/nodejs/node/pull/29968
PR-URL: https://github.com/nodejs/node/pull/29912


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>