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/securejoin.rs

@@ -861,7 +861,8 @@ fn encrypted_and_signed(
         } else {
             warn!(
                 context,
-                "Message does not match expected fingerprint {expected_fingerprint}.",
+                "Message does not match expected fingerprint {}.",
+                expected_fingerprint.human_readable()
             );
             false
         }