Fix user display name permission check

What does this MR do?

Fix for a bug in !48271 (merged), part of #29133 (closed).

The current_user and user were swapped in the permission check for the UsersHelper#user_display_name method, resulting in authenticated users seeing an unconfirmed user's full name on the profile page.

This issue would have been caught if I had the authenticated user branch of the feature spec, which I've now added. Whoops!

This change does not include a changelog since it falls under the exception:

A fix for a regression introduced and then fixed in the same release (i.e., fixing a bug introduced during a monthly release candidate) should not have a changelog entry.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
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• 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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If 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 and @ mention @gitlab-com/gl-security/appsec
• [-] 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