Fix the default branches sorting to actually be 'Last updated'

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2931

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #26890 (closed)