[mongodb][hotfix] Fix MongoDB's rewriteOutputBuffer to emit +I rather than +U (#2760) (#2760)

flink-cdc-connect/flink-cdc-source-connectors/flink-connector-mongodb-cdc/src/main/java/com/ververica/cdc/connectors/mongodb/source/reader/fetch/MongoDBFetchTaskContext.java

@@ -166,6 +166,8 @@ public class MongoDBFetchTaskContext implements FetchTask.Context {
                 case INSERT:
                 case UPDATE:
                 case REPLACE:
+                    value.put(
+                            MongoDBEnvelope.OPERATION_TYPE_FIELD, OperationType.INSERT.getValue());
                     outputBuffer.put(key, changeRecord);
                     break;
                 case DELETE: