Fix rails command to find pending and failed batch migrations

What does this MR do?

Currently, the documented way to find if there are any batch background migrations from the command line is to check puts Gitlab::Database::BackgroundMigrationJob.pending.count, this returns non-zero numbers even when no batch background migrations are pending.

This MR replaces that command with one which will check puts Gitlab::Database::BackgroundMigration::BatchedMigration.queued.count, which is what is used for the dashboard: https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/controllers/admin/background_migrations_controller.rb#L11.

It also adds a command to check for failed batch background migrations. Unfortunately, this changed between version 14.9 and 14.10. So, two examples are provided. One for each set of versions.

Related issues

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