refactor: Make Fingerprint not implement Display (#8177)

Currently, the Fingerprint type implements Display, but this doesn't get
you the canonical fingerprint representation, but something
human-readable. This is confusing, and back when I first used
`Fingerprint`, I immediately wrote a bug because of this. So, instead,
make a function `human_readable()` on Fingerprint.

This comes from the discussion at
https://github.com/chatmail/core/pull/8174#discussion_r3143130722.

src/qr/qr_tests.rs

@@ -388,7 +388,7 @@ async fn test_decode_openpgp_fingerprint() -> Result<()> {
         bob,
         &format!(
             "OPENPGP4FPR:{}#a=alice@example.org",
-            alice_contact.fingerprint().unwrap()
+            alice_contact.fingerprint().unwrap().hex()
         ),
     )
     .await?;