Only support single scope argument.

We can extend this later, but for now we don't have the use case.

app/models/concerns/atomic_internal_id.rb

@@ -28,11 +28,9 @@ module AtomicInternalId
     def has_internal_id(on, scope:, usage: nil, init:) # rubocop:disable Naming/PredicateName
       before_validation(on: :create) do
         if self.public_send(on).blank? # rubocop:disable GitlabSecurity/PublicSend
-          scope_attrs = [scope].flatten.compact.each_with_object({}) do |e, h|
-            h[e] = self.public_send(e) # rubocop:disable GitlabSecurity/PublicSend
-          end
-
+          scope_attrs = { scope => self.public_send(scope) } # rubocop:disable GitlabSecurity/PublicSend
           usage = (usage || self.class.table_name).to_sym
+
           new_iid = InternalId.generate_next(self, scope_attrs, usage, init)
           self.public_send("#{on}=", new_iid) # rubocop:disable GitlabSecurity/PublicSend
         end