Update ForEach to ParallelForEach in OrderBatchProcessor

The commit changes the 'ForEach' activity to 'ParallelForEach' in the 'OrderBatchProcessor.cs' file. This adjustment seeks to iterate through orders in parallel rather than sequentially, aiming to improve efficiency and performance. The 'ItemSource' property was also renamed to 'Items' to align with the new activity.

src/samples/aspnet/Elsa.Samples.AspNet.BatchProcessing/Workflows/OrderBatchProcessor.cs

@@ -17,9 +17,9 @@ protected override void Build(IWorkflowBuilder builder)
             {
                 new WriteLine("Fetching orders..."),
                 new FetchOrders(),
-                new ForEach<Order>
+                new ParallelForEach<Order>
                 {
-                    ItemSource = new(orders) 
+                    Items = new(orders) 
                 },
                 new WriteLine("Done!")
             }