Update graceful shutdown sample

The sample [that shows graceful shutdown](https://github.com/spring-projects/spring-batch/blob/main/spring-batch-samples/src/test/java/org/springframework/batch/sample/GracefulShutdownFunctionalTests.java) uses `JobExecution#stop` which has been deprecated in #1605 . This sample should be updated to use `JobOperator#stop`.

Related to #3663 .