Undeprecated migration for bibseries and imprint within bibref problematic

[rockivist]

The undeprecated migration of bibseries and imprint within bibref is problematic. Because in some contexts bibref is converted into ref, and in other instances the migration keeps bibref but shifts all of the linking attributes to a child ref element, any bibseries or imprint elements within bibref wind up within ref, where they are not allowed.

I'm confident this is a relative edge case - I just noticed it when I was testing updates to the migration style sheets. I'm going to leave the migration as is. If somebody needs to use the undeprecated migration, and if that person needs to use bibseries or imprint within bibref, they will have to clean it up manually.

Just logging this issue so there is a record of it.