Fix incorrect background job scheduling

What does this MR do and why?

Gitlab::Database::Migrations::BackgroundMigrationHelpers#queue_background_migration_jobs_by_range_at_intervals schedules jobs too far in to the future. This is because EachBatch yields an index starting at 1 rather than 0.

In short, if you schedule a job with a delay_interval of X.minutes, then the first job will be scheduled to begin in X.minutes rather than immediately at 0.minutes. The spec changes in this MR demonstrate this well.

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