Allow overriding scaffolded many-to-many mapping in OnModelCreatingPartial

[ajcvickers]

The shared Dictionary mapping for many-to-many generated by scaffolding cannot be overridden with a concrete join type in the model building partial without also deleting the existing mapping from OnModelCreating. See https://docs.microsoft.com/en-us/ef/core/what-is-new/ef-core-6.0/breaking-changes#many-to-many-relationships-without-mapped-join-entities-are-now-scaffolded