Fix warning on Rails 7

Rails 7 deprecates ActiveSupport's `Enum#sum` in favor of Ruby's one,
which requires an initial argument when the elements are not numeric.
Here, it's easier to just use `Enum#flatten`.

spec/lib/solidus_subscriptions/subscription_generator_spec.rb

@@ -75,7 +75,7 @@
         weekly_subscriptions,
         expiring_subscriptions,
       ]
-      grouping_result = described_class.group(subscriptions.sum)
+      grouping_result = described_class.group(subscriptions.flatten)
 
       expect(grouping_result).to match_array(subscriptions)
     end