Free `system` namespace by moving uploads into `uploads/-`

What does this MR do?

In this MR we move all uploads in the #{Rails.root}/uploads/system directory into #{Rails.root}/uploads/-/system.

That way we can free up the system group name.

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

The background migration that renames all the uploads.path-entries.

I'll dig up some numbers for that.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Adjusted for this feature/bug
  • All builds are passing

What are the relevant issue numbers?

Closes #33550 (closed)