Prevent changes on synced epic work item

When a work item is synced to a legacy epic it should not be possible to change that epic WI.

There are several ways currently to do that:

• Work Item side
  • Convert graphql mutation using the app/graphql/mutations/work_items/convert.rb
  • Quick action /type
  • Quick action /promote_to
  • Delete mutation /app/graphql/mutations/work_items/delete.rb
• Issue side
  • REST
    • Update issue
      • direct type update
      • use quick actions to set type
    • delete issue
  • GraphQL
    • Update mutation
      • direct type update
      • use quick actions to set type
    • delete mutation
• ?