Uses non-feature-gated GitLab team member check for gitlab_service_user?

What does this MR do?

This MR changes the gitlab_service_user? function to use a non-feature-gated check to see if the service user belongs to GitLab.

!45310 (merged) implemented the gitlab_service_user? check, but the check to see if the user belonged to GitLab was feature-gated by the :gitlab_employee_badge feature flag.

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
• 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
• 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