Pass demodulize class name for BackgroundMigrationWorker

What does this MR do?

Pass demodulize class name for BackgroundMigrationWorker

Since !18048 (merged), we no longer accept constants outside of Gitlab::BackgroundMigration

Related issue: #30229 (closed)

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.