Container registry batched background migrations

Suleimi Ahmedrequested to merge
container-registry-batched-background-migrations into master

Related to Container Registry: Batched Background Migrations (#122 - closed) and Plan Enabling BBM on Gitlab.com (Production rea... (gitlab-org/container-registry#1287 - closed).

This is the readiness review for the Beta (and Experiment) maturity levels of this feature. The General Availability maturity level review will come later once we're confident about the new feature.

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