Store bare class name for batched migrations

What does this MR do?

Relates to #323768 (closed)

As a follow-up from the previous MR !54628 (merged), change the batched_background_migrations table to only store the unqualified class name for both job_class_name and batch_class_name.

This gives more flexibility and leaves it up to the application to look for the classes in the correct location(s). If the classes are moved into another module, or we introduce new modules in the future, we won't need to update the data in the database.

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