Adds Admin::BatchedBackgroundMigrations endpoints to OpenApiV2 doc

What does this MR do and why?

• Adds API::Admin::BatchedBackgroundMigrations endpoints to OpenApiV2 documentation following the instruction from this issue.
  • Adds proper success and failure cases
  • Sets example value for the attributes exposed.

Screenshots or screen recordings

Expanding a specific API:

How to set up and validate locally

1. rake gitlab:openapi:generate
2. Copy the generated openapi_v2.yaml file o/p to swagger online editor
3. Navigate to batched_backgrond_migrations tag to see the changes.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Closes #376856