src: use non-deprecated V8 microtasks API

SetAutorunMicrotasks is deprecated and a warning will be emitted
starting from V8 6.5. Use the non-deprecated SetMicrotasksPolicy API
instead.

PR-URL: https://github.com/nodejs/node/pull/18753


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Yang Guo <yangguo@chromium.org>