Allow to sort by project size on admin projects page

What does this MR do?

Restores dropdown option Largest repository on admin projects page

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

Check if admin_projects_sort_options_hash is really needed. The option to sort by Largest repository was only present on the admin area, when this section got redesigned I wanted to re-use the shared dropdown partial but the Largest repository option was missing. Since /admin/projects have this additional sorting option I thought is convenient to have another hash options for the admin area. Another approach would have been to allow to sort by Largest repository everywhere else but that will require design work for other views.

Why was this MR needed?

Because of missing option reported by user.

What are the relevant issue numbers?

Fixes #20317 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together Closes #20317 (closed)