Run batched migrations on self-managed instances

What does this MR do?

This enables batched background migrations for all instances. We need them to be completed before we can swap columns to bigint.

Without this cron job running, upgrading to a version that has the bigint swap migrations would fail.

See #288005 (comment 614744621) / gitlab-org/charts/gitlab#2785 (closed) / #334996 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

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