Add more docs about background upload removal

This follows up on a number of issues we found when direct uploads
became the default:

- Buckets may not have delete permissions
- Using a bucket prefix no longer works

We include a workaround to make background uploads work again.

Relates to items discussed in:
- https://gitlab.com/gitlab-org/gitlab/-/issues/365810
- https://gitlab.com/gitlab-org/gitlab-runner/-/issues/26869