Remove inappropriate batch size in requeue background migration jobs

What does this MR do and why?

Noticed while I was working in !71700 (merged).

BATCH_SIZE const in BG migration represents a batch size per background migration job. You can see this line:

BATCH_SIZE = 1_000 # Number of rows to process per job

However, in requeue_background_migration_jobs_by_range_at_intervals context, this batch_size is used for loading rows from background_migration_jobs table. Meaning this query performance implication is same with any BG migrations, hence we should use the internal batch size, instead of allowing callers to change the batch size.

Please see this MR for more context.

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