[keyserver] Set max-allowed-packet for mysqldump backups

Summary:
We already set this MariaDB config param on the server side in keyserver/docker-compose.yml.

In ENG-3626, I identified that mysqldump defaults to a lower 16 MiB limit, and we have to set this increased limit there too.

Test Plan:

1. I patched my production keyserver with this diff
2. So I didn't have to wait for the next backup, I patched my production keyserver to generate a backup at the current time
3. I took the generated backup and I restored it on my local machine's Docker environment using this patch: https://gist.github.com/Ashoat/24590ff90441b1d2d4e5e2bf7440d818
4. I ran the Docker container locally and confirmed I was able to log into the web app and play around, and the data looked correct

Reviewers: atul, tomek

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D7368