Use ServiceResponse in WebHookService

What does this MR do and why?

This is small refactor to use ServiceResponse in WebHookService. It should have no functional change.

The response is used in WebHooks::HookExecutionNotice and that code requires no change:

ServiceResponse.success(message: 'Foo', http_status: 200).to_h => {:status=>:success, :message=>"Foo", :http_status=>200, :reason=>nil}

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.