Deprecate PowerConverter based to_polymorphic_type

This was very sparsely used. Instead of porting it elsewhere, just cast the one
caller individually.

If folks are using this downstream (I'd guess they aren't) we could consider
putting this mapper at the top `Hyrax` level, or somewhere similar.

app/conversions/power_converters/polymorphic_type.rb

@@ -1,4 +1,5 @@
 PowerConverter.define_conversion_for(:polymorphic_type) do |input|
+  Deprecation.warn('PowerConverter is deprecated. Use `.base_class` instead')
   if input.respond_to?(:base_class)
     input.base_class
   elsif input.is_a?(ActiveRecord::Base)

app/models/sipity/workflow_action.rb

@@ -34,7 +34,7 @@ def self.name_for(input, &block)
                when Sipity::WorkflowAction
                  input.name
                end
-      Sipity.handle_conversion(result, &block)
+      Sipity.handle_conversion(input, result, :to_sipity_action_name, &block)
     end
   end
 end

app/services/hyrax/workflow/permission_query.rb

@@ -242,7 +242,7 @@ def scope_entities_for_the_user(user:)
       def scope_users_for_entity_and_roles(entity:, roles:)
         entity = Sipity::Entity(entity)
         role_ids = Array.wrap(roles).map { |role| Sipity::Role(role).id }
-        user_polymorphic_type = PowerConverter.convert_to_polymorphic_type(::User)
+        user_polymorphic_type = ::User.base_class
 
         user_table = ::User.arel_table
         agent_table = Sipity::Agent.arel_table