process: change default --unhandled-rejections=throw

This is a semver-major change that resolves DEP0018.

All users that have set an unhandledRejection hook or set a non-default
value for the --unhandled-rejections flag will see no change in behavior
after this change.

Refs: https://nodejs.org/dist/latest/docs/api/deprecations.html#deprecations_dep0018_unhandled_promise_rejections

PR-URL: https://github.com/nodejs/node/pull/33021
Fixes: https://github.com/nodejs/node/issues/20392


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Mary Marchini <oss@mmarchini.me>
Reviewed-By: Shelley Vohr <codebytere@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>