[lib][native][web][landing] Silence unused-promise using void keyword

Summary:
The Flow team recommends using the void keyword to silence the unused-promise Lint. See the "Fire-and-forget" part of this blog post.

In this diff, I add the void keyword to a bunch of places in native, web, landing, and the parts of lib that are only used from native/web/landing. In these contexts it's safe to ignore promise rejections, because unlike Node.js there are no negative consequences to it. In contrast, we need to be more careful in keyserver (and potentially desktop?) because Node.js will crash when it encounters an unhandled promise rejection.

Depends on D10175

Test Plan: Flow

Reviewers: michal

Reviewed By: michal

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10176