Fix deprecation warning in ContainerRepository

What does this MR do?

Running spec/services/groups/transfer_service_spec.rb, I've found this deprecation warning:

DEPRECATION WARNING: Class level methods will no longer inherit scoping from `for_group_and_its_subgroups` in Rails 6.1. To continue using the scoped relation, pass it into the block directly. To instead access the full set of models, as Rails 6.1 will, use `ContainerRepository.default_scoped`. (called from block in <class:ContainerRepository> at gitlab/app/models/container_repository.rb:29)

This removes the deprecation warning.