Fix SSH mirrors not working in FIPS mode

What does this MR do and why?

Previously on a FIPS system, adding a project mirror via SSH would fail because the ssh_data gem attempted to generate the MD5 hash (https://github.com/github/ssh_data/blob/1c4cacb58fe1cadef31639f5f892c5738cc51be6/lib/ssh_data/public_key/base.rb#L19) of the server's public keys. To fix this, we return the SHA256 hash in the controller and make the frontend prefer this when it is available.

Relates to #364562 (closed)

Screenshots or screen recordings

Before (FIPS system)

Before (non-FIPS system)

After

How to set up and validate locally

1. Go to a project -> Settings -> Repository.
2. Under Mirroring Repositories, enter ssh://git@gitlab.com/gitlab-org/gitlab-test.git or any SSH URL.
3. Click on Detect host keys.

On a FIPS system, this fails with the screenshot above.

On a standard or a FIPS system, we see the SHA256 hashes.

MR acceptance checklist

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