[AIRFLOW-5608] Gracefully stop executor when SIGTERM is received by SchedulerJob (#6274)
[AIRFLOW-5608] Gracefully stop executor when SIGTERM is received by SchedulerJob (#6274)

(cherry picked from commit 7c9b44dbfc72df951aadbda006229977beb6d47c)

owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow