Allow date parameters on Issues, Notes, and Discussions API for group owners

Continuation of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15765 - thanks @fldubois!

What does this MR do?

This allows group owners to set created_at and iid in the API. Previously, we only let admins or project owners do this, which was inconsistent.

What are the relevant issue numbers?

Closes #40059 (closed).

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides