Clarify naming of `WorkItems::Widgets::*Service::*Service`

The following discussion from !92864 (merged) should be addressed:

• @splattael started a discussion:

  Question (non-blocking)

  This service does not have an execute method as suggested in https://docs.gitlab.com/ee/development/reusing_abstractions.html#service-classes:

  In Service classes the use of execute and #execute is preferred over call and #call.

  And it also does not return ServiceResponse which described in https://docs.gitlab.com/ee/development/reusing_abstractions.html#serviceresponse:

  Service classes usually have an execute method, which can return a ServiceResponse.

  It seems that this "service" is used by WorkItems::UpdateService internally and is not a real service. I was wondering if we could move this functionality into lib/ and use a different terminology to avoid confusion.

  Just a thought. I'll create an issue to discuss.