Support .Net framework 2.0 (#1062)

scripts/dotnet/OnlineRecognizer.cs

@@ -3,7 +3,6 @@
 /// Copyright (c)  2024.5 by 东风破
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using System.Runtime.InteropServices;
 
 namespace SherpaOnnx
@@ -49,7 +48,15 @@ namespace SherpaOnnx
         // The caller should ensure all passed streams are ready for decoding.
         public void Decode(IEnumerable<OnlineStream> streams)
         {
-            IntPtr[] ptrs = streams.Select(s => s.Handle).ToArray();
+            // TargetFramework=net20 does not support System.Linq
+            // IntPtr[] ptrs = streams.Select(s => s.Handle).ToArray();
+            List<IntPtr> list = new List<IntPtr>();
+            foreach (OnlineStream s in streams)
+            {
+              list.Add(s.Handle);
+            }
+
+            IntPtr[] ptrs = list.ToArray();
             Decode(_handle.Handle, ptrs, ptrs.Length);
         }