find_user users helper method no longer overrides find_user API helper method.

What does this MR do?

Renames the API::Users#find_user method that was overriding the find_user method of the helper when using SUDO

This can be better visually explained by looking at:

• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/api/users.rb#L14 (Users helper method)
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/api/helpers.rb#L69 (API helper method)
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/api/helpers.rb#L426 (Place where the overriding was happening)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #37467 (closed)