net: use `_final` instead of `on('finish')`

Shutting down the connection is what `_final` is there for.

PR-URL: https://github.com/nodejs/node/pull/18608


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