Move profiles/keys#get_keys to users#ssh_keys on Web UI with keeping the feature publicly accessible

Once !35507 (merged) moved profiles/keys#get_keys to users#ssh_keys on Web UI (%13.2), but it changed the behavior for non-logged in users, !42288 (merged) reverted that to keep the feature publicly accessible.

For code maintainability, it is still better to move profiles/keys#get_keys to users#ssh_keys on Web UI with keeping the feature publicly accessible.

Why?

As the API for List SSH keys for user has /users/:id_or_username/keys, we should align this Web UI endpoint with the counterpart API endpoint.

security & ~"technical debt": All Profiles::*Controllers are designed only for operations the logged-in and own user. Having skip_before_action :authenticate_user! to some action there can potentially be a security hole.