Refactor weight service to use callback classes
What does this MR do and why?
Move the work item weight widget service to the new callback classes architecture.
Related to #412801 (closed)
More info in https://docs.gitlab.com/ee/development/work_items_widgets.html#widget-callbacks
MR acceptance checklist
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How to set up and validate locally
Tests in ee/spec/requests/api/graphql/mutations/work_items/update_spec.rb
validate that the update mutation still works as before.
Edited by Heinrich Lee Yu