SLA miss alerting for mapped tasks

[lyanita]

With SLA rework on Airflow's roadmap, I would like to have SLA miss alerting available for mapped tasks. Currently, we run into this exception where we have to set sla=None whenever we have a mapped task: https://github.com/apache/airflow/blob/main/airflow/models/mappedoperator.py#L328.

This makes SLA miss alerting pointless when not every task in a DAG can be tracked for SLA misses.