[Connector Builder] Set default value for end_datetime (#21995)

* set default value for end_datetime

* set default format as well

Author: lmossman

airbyte-webapp/src/components/connectorBuilder/Builder/StreamSlicerSection.tsx

@@ -82,9 +82,9 @@ export const StreamSlicerSection: React.FC<StreamSlicerSectionProps> = ({ stream
       label: "Datetime",
       typeValue: "DatetimeStreamSlicer",
       default: {
-        datetime_format: "",
+        datetime_format: "%Y-%m-%d %H:%M:%S.%f+00:00",
         start_datetime: "",
-        end_datetime: "",
+        end_datetime: "{{ now_utc() }}",
         step: "",
         cursor_field: "",
       },