[keyserver] Silence unused-promise Flow lint in backupDB

Summary:
The async IIFE in backupDB contains two try-catch blocks that guarantee it will never reject.

As such, we're safe to ignore the unused-promise Flow lint. Our primary reason for enabling that Lint is to catch unhandled promise rejections within keyserver, but there is no potential of that here.

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

Depends on D10177

Test Plan: Flow

Reviewers: michal

Reviewed By: michal

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10178