Prefer ActiveModel::Errors#add over <<

Ref:
https://api.rubyonrails.org/v6.1.4/classes/ActiveModel/Errors.html#method-i-add

app/forms/waste_carriers_engine/base_form.rb

@@ -71,7 +71,7 @@ def transient_registration_valid?
       return if transient_registration.valid?
 
       transient_registration.errors.each do |_attribute, message|
-        errors[:base] << message
+        errors.add(:base, message)
       end
     end