[BUG] Add cancellation_time_millis to resolve Strict Dynamic Mappin…

…g issue in .tasks index

- Fixed issue where `.tasks` index failed to update due to StrictDynamicMappingException when a task was cancelled.
- Added missing `cancellation_time_millis` field to `task-index-mapping.json`.
- Ensured proper handling of task cancellation events in Cross-Cluster Replication (CCR) by updating the mappings.
- Verified by creating and deleting an auto follow rule without StrictDynamicMappingException.

server/src/main/resources/org/opensearch/tasks/task-index-mapping.json

@@ -34,6 +34,9 @@
           "start_time_in_millis": {
             "type": "long"
           },
+          "cancellation_time_millis": {
+            "type": "long"
+          },
           "type": {
             "type": "keyword"
           },