Fix caching of trending projects

What does this MR do?

This fixes the caching of trending projects.

Why was this MR needed?

An ActiveRecord::Relation was being cached instead of an Array of rows.

Does this MR meet the acceptance criteria?

• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/23097#note_16654556