[keyserver] Use VALUE instead of VALUES in DUPLICATE KEY UPDATE on MariaDB

Summary:
See here – MariaDB 10.3.3 and higher require this change.

Depends on D4522

Test Plan: I ran the web application and clicked around a bunch. Full test plan will come later

Reviewers: jonringer-comm, atul, palys-swm

Reviewed By: atul

Subscribers: Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4528