[keyserver] Modify filename column in MariaDB uploads table to utf8

Summary:
Instead of just updating the filename column, I updated the default to utf8_mb4 (the "real" utf8 encoding in MariaDB). That way if we create a future column, it will default to utf8.

I think added explicit annotations to keep the other columns the same, to avoid any weird effects.

Test Plan:

1. I tested that the migration works in my local environment
2. I tested that the setup-db.js query works by copy-pasting it into a MariaDB console in my local environment (with a different name for the table)
3. I then ran SHOW FULL COLUMNS FROM on both the migration result and the setup-db.js result to confirm they were the same: https://gist.github.com/Ashoat/0494344fa432995cf0219aed25fcb321
4. Finally, I confirmed that my repro (drag-n-drop a screenshot on macOS before it saves to desktop) no longer repro'd

Reviewers: atul, bartek

Reviewed By: atul, bartek

Subscribers: tomek, wyilio

Differential Revision: https://phab.comm.dev/D9403