fix: mark MultiUrlPickerValueConverter with DefaultValueConverter (#13347)

Co-authored-by: georgebid <[email protected]>

Author: rasmusjp

src/Umbraco.Infrastructure/PropertyEditors/ValueConverters/MultiUrlPickerValueConverter.cs

@@ -17,6 +17,7 @@
 
 namespace Umbraco.Cms.Core.PropertyEditors.ValueConverters;
 
+[DefaultPropertyValueConverter(typeof(JsonValueConverter))]
 public class MultiUrlPickerValueConverter : PropertyValueConverterBase, IDeliveryApiPropertyValueConverter
 {
     private readonly IJsonSerializer _jsonSerializer;