[AIRFLOW-6159] Change logging level of the heartbeat message to DEBUG

[kaxil]

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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"
  • https://issues.apache.org/jira/browse/AIRFLOW-6159

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  As mentioned in https://stackoverflow.com/questions/59142508/airflow-disable-heartbeat-logs , the logs containing heartbeat details can become too noisy.

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[madison-ookla]

Thanks, this has been a big issue for us. #6553 removes this block entirely which would be an alternative, but this will help for now until that gets merged.