Fix deprecation warning related to Rails 6.2

> You are passing a block expecting two parameters,
> so the old hash behavior is simulated. As this is deprecated,
> this will result in an ArgumentError in Rails 6.2.
> DEPRECATION WARNING: Enumerating ActiveModel::Errors as a hash has been deprecated.
> In Rails 6.1, `errors` is an array of Error objects,
> therefore it should be accessed by a block with a single block
> parameter like this:
>
> person.errors.each do |error|
>   attribute = error.attribute
>   message = error.message
> end

app/models/solidus_paypal_braintree/transaction.rb

@@ -18,8 +18,8 @@ class Transaction
         errors.add(:payment_method, 'Must be braintree')
       end
       if address && !address.valid?
-        address.errors.each do |field, error|
-          errors.add(:address, "#{field} #{error}")
+        address.errors.each do |error|
+          errors.add(:address, "#{error.attribute} #{error.message}")
         end
       end
     end