Changing server.pgp.upstream_url doesn't override cached pgp key

[lorienhu]

Fleet Server serves a PGP key to agents at /api/agents/upgrades/<version>/pgp-public-key.
The key is resolved as cache, then <pgp.dir>/default.pgp on disk, then a fetch from
server.pgp.upstream_url, and the fetched key is written to disk.

Once default.pgp exists on disk, changing server.pgp.upstream_url has no effect. This file is always read if it exists and never cleared.

Found while investigating https://github.com/elastic/sdh-beats/issues/7564.