Only load active payment methods in the backend

This was the original behaviour in Solidus < 2.1, but the active scope
got lost in solidusio#1540. Payment methods used to use the `available` filter
which was scoped to active payment methods only. The `available_to_admin`
scope isn't limited active payment methods, so we need to add it here.

backend/app/controllers/spree/admin/payments_controller.rb

@@ -78,7 +78,7 @@ def object_params
 
       def load_data
         @amount = params[:amount] || load_order.total
-        @payment_methods = Spree::PaymentMethod.available_to_admin
+        @payment_methods = Spree::PaymentMethod.active.available_to_admin
         if @payment && @payment.payment_method
           @payment_method = @payment.payment_method
         else