View all issues of all subgroups on the group issue page

Not included

• API changes will be done in https://gitlab.com/gitlab-org/gitlab-ce/issues/42443

What does this MR do?

On the group page it displays not only issues/merge requests of the group in request but also issues of all subgroups

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

We display only group issues now

Screenshots (if relevant)

Issues

Merge Requests

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #30106 (closed), Closes #39388 (closed)