Add a change_belongs_to Migrator method

We currently have `rename_belongs_to` and `remove_belongs_to`, but say you originally set your `on_delete: :do_nothing`, but wanted to update that to `on_delete: :cascade`. It would be great to have a `change_belongs_to` macro to handle doing that.

Locate the existing `rename_belongs_to` and `remove_belongs_to` migrator methods and read how their behavior is tested. Add the requested `change_belongs_to` capability for updating `on_delete`, including the transition from `:do_nothing` to `:cascade`, and verify that the relevant migration tests cover the new behavior.

Written by the indexing model from the issue text.