[keyserver] Bump MariaDB --innodb-buffer-pool-size config to 14000M in docker-compose.yml

Summary:
I noticed that I have this change on my keyserver running in production. There's been some recent discussion about this config in D12322 (see diff description).

The context about this change being on my production keyserver is here on Linear. For whatever reason I never got around to diffing it up, probably because the performance improvement seem minimal and I didn't have a strong argument for it.

Test Plan: See context Linear link above; I tested this and observed a slight performance improvement. I vaguely recall testing how long eg. 3 attempts of the same fetchServerThreadInfos query would take, before and after the change

Reviewers: will, tomek

Reviewed By: will

Differential Revision: https://phab.comm.dev/D12337