Adjust created field of Tobira harvest response for events

This changed in OC 16 with the Solr/Opensearch swap as far as I can see,
but it needs to be in a specific format in order to be harvested by
Tobira.

No version bump since this only restores old behaviour and is not a breaking
change.

modules/tobira/src/main/java/org/opencastproject/tobira/impl/Item.java

@@ -50,6 +50,7 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import java.time.Instant;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Date;
@@ -155,7 +156,9 @@ class Item {
           Jsons.p("title", title),
           Jsons.p("partOf", event.getDublinCore().getFirst(DublinCore.PROPERTY_IS_PART_OF)),
           Jsons.p("description", event.getDublinCore().getFirst(PROPERTY_DESCRIPTION)),
-          Jsons.p("created", event.getDublinCore().getFirst(DublinCore.PROPERTY_CREATED)),
+          Jsons.p("created", Date.from(
+            Instant.parse(event.getDublinCore().getFirst(DublinCore.PROPERTY_CREATED))
+          ).getTime()),
           Jsons.p("startTime", period.map(p -> p.getStart().getTime()).orElse(null)),
           Jsons.p("endTime", period.map(p -> p.getEnd().getTime()).orElse(null)),
           Jsons.p("creators", Jsons.arr(new ArrayList<>(creators))),