Ref System.Memory for net462 and netstandard2.0 (#1417)

* Ref System.Memory for net462 and netstandard2.0.
Use System.Buffers.Binary.BinaryPrimitives instead of Renci.SshNet.Common.Pack

* Remove unnecessary `AsSpan()`

* Apply suggestions from code review

* that's what I get for being lazy

---------

Co-authored-by: Rob Hague <rob.hague00@gmail.com>
Co-authored-by: Robert Hague <rh@johnstreetcapital.com>

Author: scott-xu

src/Renci.SshNet/Common/DerData.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Buffers.Binary;
 using System.Collections.Generic;
 
 namespace Renci.SshNet.Common
@@ -198,7 +199,8 @@ public void Write(bool data)
         /// <param name="data">UInt32 data to write.</param>
         public void Write(uint data)
         {
-            var bytes = Pack.UInt32ToBigEndian(data);
+            var bytes = new byte[sizeof(uint)];
+            BinaryPrimitives.WriteUInt32BigEndian(bytes, data);
             _data.Add(Integer);
             var length = GetLength(bytes.Length);
             WriteBytes(length);