Add queued_migration_version to facilitate BBM dependencies

What does this MR do and why?

This adds queued_migration_version attr to batched_background_migrations table, which will be used by later migrations to establish the dependencies.

More details can be found in #424881 (closed)'s description.

Rationale for choosing migration version:

Version of the migration which enqueued the BBM has been chosen as the unique key for each batched_background_migration, so that it'll be easier to establish the dependencies (instead of mentioning all columns in the existing uniq index) while writing future migrations.

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Before	After

How to set up and validate locally

1. Run migrations to create the necessary attribute AddQueuedMigrationVersionToBatchedBackgroundMigrations and AddLimitToQueuedMigrationVersionInBatchedBackgroundMigrations.

2. Enqueue a dummy batched background migration

   bundle exec rails g batched_background_migration dummy_batched_migration --table_name=projects --column_name=id --feature_category=database

3. The post migration which calls queue_batched_background_migration should have the correct queued_migration_version value specified.

4. On running the created dummy migration locally, queued_migration_version attr will get saved in the batched_background_migrations record.

   bbm = Gitlab::Database::BackgroundMigration::BatchedMigration.last
   bbm.queued_migration_version

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.

Related to #424881 (closed)