api: Allow non-admin user to get GPG public keys

What does this MR do?

Removes the requirement of administration privileges for the endpoint

GET /users/:id/gpg_keys

With this change the scope read_user is now required.

#21584 (closed)

Screenshots

No visual changes/effects.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

I believe that exposing public GPG keys for tokens that have the read_user permission does not pose a significant security threat. GitHub, for example, even exposes them publicly: https://github.com/m-bymike.gpg

@gitlab-com/gl-security/appsec, this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security -> I cannot label the MR.
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

P.S.: This is my first MR for GitLab, please excuse newbie issues...