Group::ChildrenController#index.json sort order

What does this MR do?

Specifies the sort order by updated_at of the projects shown in Group::ChildrenController#index.json.

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

Why was this MR needed?

To show projects (and subgroups) on group's top page in reverse order of their last update.

Screenshots (if relevant)

before	after
(projects sorted by project_id DESC)	(projects sorted by updated_at DESC)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• [n/a] Internationalization required/considered
• [n/a] End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #46554 (moved)