Fix batched background migration to backfill missing search data

We had to pause the batched background migration to backfill missing search data for #361219 (closed).

The reason is that because of batching_scope: -> (relation) { Issue }, each batched job instead of breaking down the work in sub-batches, starts iterating issues table from the start, until eventually gets terminated with Sidekiq::Shutdown. The related Slack discussion is here - https://gitlab.slack.com/archives/CNZ8E900G/p1657161519642019?thread_ts=1656578448.257079&cid=CNZ8E900G.

Specifying batching_scope is needed when we have additional filters and not iterating by id only. There is some work in progress to improve BatchedMigrationJob's API, and we should add more docs/examples to make it easier to understand and use.

In order to complete this migration we need new MR that will:

• Fix lib/gitlab/background_migration/backfill_imported_issue_search_data.rb
• Make db/post_migrate/20220621040800_backfill_imported_issue_search_data.rb a no-op
• Reschedule the migration by deleting the old one and enqueuing a new migration

Here is an example MR of rescheduling a batched migration - !87496 (diffs).