GitLab upgrade assistant should check for running migrations

We should warn a customer to avoid upgrading if there are currently Advanced Search migrations running in the background and haven't completed yet.

If ::Elastic::DataMigrationService.pending_migrations has any items, we should fail the check to encourage self managed customers to let Advanced Search migrations complete before upgrading. This is not a major problem between minor versions of GitLab, but it is a large problem between major versions because we deprecate migrations at that time.

Steps to implement this would be:

1. Create a check called AdvancedSearchMigrationsCheck, similar to SearchCheck
2. Add check to app_task.rb here