dont use generic version of PtrToStructure

src/csharp/Grpc.Core/Internal/AuthContextSafeHandle.cs

@@ -74,7 +74,11 @@ namespace Grpc.Core.Internal
 
         private AuthProperty PtrToAuthProperty(IntPtr authPropertyPtr)
         {
-            var nativeAuthProperty = Marshal.PtrToStructure<NativeAuthProperty>(authPropertyPtr);
+            #pragma warning disable 0618
+            // We need to use the obsolete non-generic version of Marshal.PtrToStructure, because the generic version is not available
+            // in net45 on Windows.
+            var nativeAuthProperty = (NativeAuthProperty) Marshal.PtrToStructure(authPropertyPtr, typeof(NativeAuthProperty));
+            #pragma warning restore 0618
             var name = Marshal.PtrToStringAnsi(nativeAuthProperty.Name);
             var valueBytes = new byte[(int) nativeAuthProperty.ValueLength];
             Marshal.Copy(nativeAuthProperty.Value, valueBytes, 0, (int)nativeAuthProperty.ValueLength);