Remove explicit_mr_work_item_relations feature flag

What does this MR do?

Removes the explicit_mr_work_item_relations feature flag. The flag has been enabled by default since GitLab 19.5, so nothing changes for users.

• Backend: the GraphQL field, the two mutations, the REST related merge requests endpoints, and the work item create callback no longer check the flag.
• Frontend: the merge request sidebar always loads the linked work items and the permission to change them. The old fallback to linkedWorkItems is gone.
• Specs: the flag-off contexts are removed.

A second commit updates the docs history. That commit needs the URL of this MR.

How to test

1. Open a merge request in GDK.
2. In the right sidebar, check that Related work items shows the linked work items.
3. As a user who can change the merge request, add a work item, then remove it. Both actions work.

References

• Resolves #629734 (closed)
• Flag enabled by default in !256281 (merged)
• Feature issue: #601590 (closed)