[AIRFLOW-5714] Collect SLA miss emails only from tasks missed SLA

[milton0825]

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Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
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Description

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Currently when a task in the DAG missed the SLA, Airflow would traverse through all the tasks in the DAG and collect all the task-level emails. Then Airflow would send an SLA miss email to all those collected emails, which can add unnecessary noise to task owners that does not contribute to the SLA miss.

Thus, changing the code to only collect emails from the tasks that missed the SLA.

Tests

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