[keyserver] Add keyserver/certs to .gitignore

Summary:
When configuring APNs in the Docker environment, we set COMM_JSONCONFIG_secrets_comm_apn_config via an env var in keyserver/.env. However, the APNs config points to a .p8 file.

We could put that file in keyserver/secrets where it goes in the local dev environment, but that folder is .dockerignore'd to make it easier to use a single folder to test both the Docker environment / local dev environment. (Otherwise, the Docker environment would be pulling config from both keyserver/.env and keyserver/secrets, which might be confusing.)

Instead, on my prod keyserver I have been using this keyserver/certs folder. This diff adds it to .gitignore so it stops showing up when I type in git status. Unlike keyserver/secrets and keyserver/facts, however, keyserver/certs will NOT go into .dockerignore.

Test Plan: Make sure keyserver/certs no longer shows up in git status

Reviewers: atul, jon

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7369