Omit MD5 fingerprints in deploy key API response

In FIPS mode, MD5 fingerprints are not generated, but the SHA256
versions are available. Adjust the API entity to export the SHA256
fingerprint and schema to ensure that the SHA256 value is available.

Update deploy keys API docs to reflect this. Include `fingerprint` and
`fingerprint_sha256` where the fields were missing before. Add a note
about FIPS behavior.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/364562

Changelog: changed