Reduce buffer size used in XmlReader when using Async mode (#63459)

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

Fixes #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;