align serializer max length of attribute value with the model (#1074)

cvat/apps/engine/serializers.py

@@ -352,7 +352,7 @@ class ImageMetaSerializer(serializers.Serializer):
 
 class AttributeValSerializer(serializers.Serializer):
     spec_id = serializers.IntegerField()
-    value = serializers.CharField(max_length=64, allow_blank=True)
+    value = serializers.CharField(max_length=4096, allow_blank=True)
 
     def to_internal_value(self, data):
         data['value'] = str(data['value'])