Fix broken SLA Mechanism

[kaxil]

closes #14050

We were not de-serializing BaseOperator.sla properly, hence
we were returning float instead of timedelta object.

Example: 100.0 instead of timedelta(seconds=100)

And because we had a check in _manage_sla in SchedulerJob and DagFileProcessor,
we were skipping SLA.

SchedulerJob:

airflow/airflow/jobs/scheduler_job.py

Lines 1766 to 1768 in 88bdcfa

	if not any(isinstance(ti.sla, timedelta) for ti in dag.tasks):
	self.log.debug("Skipping SLA check for %s because no tasks in DAG have SLAs", dag)
	return

DagFileProcessor:

airflow/airflow/jobs/scheduler_job.py

Lines 395 to 397 in 88bdcfa

	if not any(isinstance(ti.sla, timedelta) for ti in dag.tasks):
	self.log.info("Skipping SLA check for %s because no tasks in DAG have SLAs", dag)
	return

Tested it with breeze too: #14050 (comment)

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[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest master at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.