agent: Use SHA-256 for SSH fingerprint by default

* agent/gpg-agent.c (parse_rereadable_options): Change default ssh
fingerprint digest.
(main): Ditto.

• Co-authored-by: Jakub Jelen <jjelen@redhat.com>
• GnuPG-bug-id: T5434
• Signed-off-by: Werner Koch <wk@gnupg.org>