No longer accept fingerprints for /internal/authorized_keys

What does this MR do?

gitlab-shell-authorized-keys does a GET request against the internal API to learn whether a given SSH key is known to GitLab and, if so, what user it is attached to. This is used to validating SSH sessions.

For a very long time, gitlab-shell has sent only the full base64 encoded public key, from which we derive a fingerprint for looking up the key on the GitLab side. However, the API does have legacy support for gitlab-shell sending just the key fingerprint. It is not used in gitlab-shell.

We want to update GitLab so it derives the SHA256 fingerprint for the search, rather than the MD5 fingerpring. Doing this is made easier if we remove the fingerprint parameter and only accept key in the API endpoint.

This is safe to do, because fingerprint is unused, and does not break any compatibility guarantees since the internal API is not bound by the v4 REST API compatibility promise.

The documentation does not refer to the fingerprint parameter at all, and doing this in %14.0 should means that we don't have to think about zero-downtime compatibility either, although that should also be fine.

Here's the current docs: https://docs.gitlab.com/ee/development/internal_api.html#authorized-keys-check

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)