Topics: order by `non_private_projects_count`

What does this MR do and why?

This MR is the second step of the issue Explore topics: improve counter consistency (#351115). In the previous MR !79140 (merged), the non_private_projects_count column was added. This MR:

• finalizes and cleans up the background migration added in !79140 (merged)
• uses non_private_projects_count instead of total_projects_count for topic ordering

🛠 with ❤ at Siemens

/cc @bufferoverflow

How to set up and validate locally

1. Visit the project settings of different projects and add/remove topics
2. Visit the Explore topics page: http://localhost:3000/explore/projects/topics

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.