Look up SSH keys by SHA256 fingerprint, not MD5 fingerprint

What does this MR do?

This change takes GitLab one step closer to being FIPS-compliant, and means git operations over SSH can work in a FIPS-enforcing environment.

GitLab Shell sends the full SSH key to GitLab, and we have the SHA256 fingerprint stored in the database already, so which fingerprint we use to look up the key is an implementation detail; the tests (correctly, IMO) don't mandate a particular form.

The next step for FIPS compliance on this feature is to stop generating MD5 fingerprints when SSH keys are added.

Screenshots or Screencasts (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #195668 (closed)