feat(registry): add registry db background migrations config

What does this MR do?

The merge request adds registry background migration config to the registry database subsection of the helm chart. The background migration feature is disabled by default in the values.yaml and again in a registry FF

Related issues

gitlab-org&13609

Author checklist

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Required

For anything in this list which will not be completed, please provide a reason in the MR discussion.

• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated, and test plan for scenarios not covered by automated tests.
• Equivalent MR/issue for omnibus-gitlab opened: gitlab-org/omnibus-gitlab#8755

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab Chart.