Return ServiceResponse from Users::Abuse::NamespaceBans::CreateService

What does this MR do and why?

As a follow up to MR feedback (see also recommendation in docs), this MR updates Users::Abuse::NamespaceBans::CreateService to return a ServiceResponse instead of a Hash.

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Validate that all automated tests are passing

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.