**Apache Airflow version**: 2.1.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): v1.18.17-gke.1901

**Environment**:

- **Cloud provider or hardware configuration**: Google Cloud
- **OS** (e.g. from /etc/os-release): Debian GNU/Linux 10 (buster)
- **Kernel** (e.g. `uname -a`): Linux airflow-scheduler-7697b66974-m6mct 5.4.89+ #1 SMP Sat Feb 13 19:45:14 PST 2021 x86_64 GNU/Linux
- **Install tools**:
- **Others**:

**What happened**:
When the `scheduler` is restarted the currently running tasks are facing SIGTERM error.
Every time the `scheduler` is restarted or re-deployed then the current `scheduler` is terminated and a new `scheduler` is created. If during this process exist tasks running the new `scheduler` will terminate these tasks with `complete` status and new tasks will be created to continue the work of the terminated ones. After few seconds the new tasks are terminated with `error` status and SIGTERM error.

<details><summary>Error log</summary> [2021-07-07 14:59:49,024] {cursor.py:661} INFO - query execution done
[2021-07-07 14:59:49,025] {arrow_result.pyx:0} INFO - fetching data done
[2021-07-07 15:00:07,361] {local_task_job.py:196} WARNING - State of this instance has been externally set to failed. Terminating instance.
[2021-07-07 15:00:07,363] {process_utils.py:100} INFO - Sending Signals.SIGTERM to GPID 150
[2021-07-07 15:00:12,845] {taskinstance.py:1264} ERROR - Received SIGTERM. Terminating subprocesses.
[2021-07-07 15:00:12,907] {process_utils.py:66} INFO - Process psutil.Process(pid=150, status='terminated', exitcode=0, started='14:59:46') (150) terminated with exit code 0 </details>

**What you expected to happen**:

The tasks currently running should be allowed to finish their process or the substitute tasks should execute their process with success.
The new `scheduler` should interfere with the running tasks.

**How to reproduce it**:

To reproduce is necessary to start a DAG that has some task(s) that take some minutes to be completed. During this task(s) processing a new deploy for `scheduler` should be executed. During the re-deploy, the current `scheduler` will be terminated and a new one will be created. The current task(s) will be completed (without finish their processing) and substituted for new ones that will fail in seconds.

**Anything else we need to know**:

The problem was not happening with Airflow 1.10.15 and it started to happens after the upgrade to Airflow 2.1.0.
airflow/jobs/scheduler_job.py
pools = models.Pool.slots_stats(session=session)
        for pool_name, slot_stats in pools.items():
            Stats.gauge(f'pool.open_slots.{pool_name}', slot_stats["open"])
            Stats.gauge(f'pool.queued_slots.{pool_name}', slot_stats[State.QUEUED])  
            Stats.gauge(f'pool.running_slots.{pool_name}', slot_stats[State.RUNNING])  
    @provide_session
    def heartbeat_callback(self, session: Session = None) -> None:
        Stats.incr('scheduler_heartbeat', 1, 1)
    @provide_session
    def adopt_or_reset_orphaned_tasks(self, session: Session = None):
        """
        Reset any TaskInstance still in QUEUED or SCHEDULED states that were
        enqueued by a SchedulerJob that is no longer running.
        :return: the number of TIs reset
        :rtype: int
        """
        self.log.info("Resetting orphaned tasks for active dag runs")
        timeout = conf.getint('scheduler', 'scheduler_health_check_threshold')
        for attempt in run_with_db_retries(logger=self.log):
            with attempt:
                self.log.debug(
                    "Running SchedulerJob.adopt_or_reset_orphaned_tasks with retries. Try %d of %d",
                    attempt.retry_state.attempt_number,
                    MAX_DB_RETRIES,
                )