Instance Configuration page now displays correct SSH fingerprints

What does this MR do?

OpenSSL::Digest was hashing the entire file instead of extracting the public key and then hashing it. This is a problem that was already solved in Gitlab::SSHPublicKey which we can leverage here.

Screenshots

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

What are the relevant issue numbers?

Closes #40636 (closed)