[keyserver] Update mysql2 to 2.3.3

Summary: Depends on D4530

Test Plan:

1. I ran the web application and clicked around a bunch. Full test plan will come later
2. Checked mysql2 release notes to make sure there weren't any breaking changes or regressions

Reviewers: jonringer-comm, atul, palys-swm

Reviewed By: atul

Subscribers: Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4531