Migrate existing bridges to the new Ci::Bridges::Downstream type

Summary

After https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10521 gets merged we'll have a new Ci::Bridges::Downstream model for downstream bridges. We should migrate existing bridges to use the new, more specific downstream bridge type instead.

Proposal

Schedule a background migration that changes the type for all existing bridges to the new type.