More verbose scheduling output

What does this MR do?

Only adds output to the scheduling part of a background migration. This is useful in testing, but it'll also help to get an estimate while we deploy on .com.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content