Reduce buffer size used in XmlReader when using Async mode

The current choice of AsyncBufferSize resulted in the character buffer in the XmlTextReader being allocated on the Large Object Heap (LOH)

Fixes dotnet#61459

src/libraries/System.Private.Xml/src/System/Xml/Core/XmlReader.cs

@@ -81,7 +81,9 @@ public abstract partial class XmlReader : IDisposable
         internal const int BiggerBufferSize = 8192;
         internal const int MaxStreamLengthForDefaultBufferSize = 64 * 1024; // 64kB
 
-        internal const int AsyncBufferSize = 64 * 1024; //64KB
+        // Chosen to be small enough that the character buffer in XmlTextReader when using Async = true
+        // is not allocated on the Large Object Heap (LOH)
+        internal const int AsyncBufferSize = 32 * 1024;
 
         // Settings
         public virtual XmlReaderSettings? Settings => null;