Use adapter for removing legacy epic links

What does this MR do and why?

Use adapter for removing legacy epic links

This is migrating one more service to use the WorkItem service instead of relying on the legacy epic services. This doesn't change any behaviour as the Epic Service is already used in most use cases, except for the REST API.

References

• #508938

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create an epic <> epic relationship
2. Use the REST API to delete it https://docs.gitlab.com/api/epic_links/#unassign-a-child-epic

Epic.last.parent_id
  Epic Load (1.1ms)  SELECT "epics".* FROM "epics" ORDER BY "epics"."id" DESC LIMIT 1 /*application:console,db_config_database:gitlabhq_development,db_config_name:main,console_hostname:ndular--20250210-YF7L9,console_username:nicolas,line:(pry):3:in `__pry__'*/
=> 15

[4] pry(main)> Epic.last.work_item_parent_link_id
  Epic Load (1.1ms)  SELECT "epics".* FROM "epics" ORDER BY "epics"."id" DESC LIMIT 1 /*application:console,db_config_database:gitlabhq_development,db_config_name:main,console_hostname:ndular--20250210-YF7L9,console_username:nicolas,line:(pry):4:in `__pry__'*/
=> 3

Important:

• For the parent you need to use the IID
• For the child you need to use the ID

DELETE /groups/:id/epics/:epic_iid/epics/:child_epic_id

curl --request DELETE \
  --header "PRIVATE-TOKEN: <token>" \
   --url "http://gdk.test:3000/api/v4/groups/gitlab/epics/9/epics/16"

{"id":16,"work_item_id":638,"iid":10,"color":"#1068bf","text_color":"#FFFFFF","group_id":105,"parent_id":15,"parent_iid":9,"imported":false,"imported_from":"none","title":"Child epic","description":null,"confidential":false,"author":{"id":70,"username":"nicolas","public_email":null,"name":"Nicolas Dular","state":"active","locked":false,"avatar_url":"https://www.gravatar.com/avatar/ba8a37e8137420c27f436c273943b83433db4188ee0caede56b93e7fba8c7a63?s=80\u0026d=identicon","web_url":"http://gdk.test:3000/nicolas"},"start_date":null,"start_date_is_fixed":false,"start_date_fixed":null,"start_date_from_inherited_source":null,"start_date_from_milestones":null,"end_date":null,"due_date":null,"due_date_is_fixed":false,"due_date_fixed":null,"due_date_from_inherited_source":null,"due_date_from_milestones":null,"state":"opened","web_edit_url":"/groups/gitlab/-/epics/10","web_url":"http://gdk.test:3000/groups/gitlab/-/epics/10","references":{"short":"\u002610","relative":"\u002610","full":"gitlab\u002610"},"created_at":"2025-11-05T19:55:44.629Z","updated_at":"2025-11-05T19:55:44.629Z","closed_at":null,"labels":[],"upvotes":0,"downvotes":0,"_links":{"self":"http://gdk.test:3000/api/v4/groups/105/epics/10","epic_issues":"http://gdk.test:3000/api/v4/groups/105/epics/10/issues","group":"http://gdk.test:3000/api/v4/groups/105","parent":"http://gdk.test:3000/api/v4/groups/105/epics/9"}}⏎

 Epic.last.parent_id
  Epic Load (1.0ms)  SELECT "epics".* FROM "epics" ORDER BY "epics"."id" DESC LIMIT 1 /*application:console,db_config_database:gitlabhq_development,db_config_name:main,console_hostname:ndular--20250210-YF7L9,console_username:nicolas,line:(pry):5:in `__pry__'*/
=> nil
[6] pry(main)> Epic.last.work_item_parent_link_id
  Epic Load (1.0ms)  SELECT "epics".* FROM "epics" ORDER BY "epics"."id" DESC LIMIT 1 /*application:console,db_config_database:gitlabhq_development,db_config_name:main,console_hostname:ndular--20250210-YF7L9,console_username:nicolas,line:(pry):6:in `__pry__'*/
=> nil

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