[keyserver] Use secrets folder for Postmark license

Summary:
I'm not sure how much the facts/secrets distinction counts now that we're not really using it in production (Docker)...

(The initial reason for the separation is that the folders can be chmod'd with separate permissions.)

Anyways, the idea is that secret info like licenses should be kept in secrets, so I figure we can move this.

Test Plan: N/A

Reviewers: atul

Reviewed By: atul

Subscribers: palys-swm, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4454