calculate_dagrun_date_fields doesn't understand how it is called

### Body It has an argument `last_automated_run` but in reality, what it is given is not always the latest run. This is because sometimes it is called, in the scheduler, with whatever run the scheduler happens to be dealing with -- which might not be the latest run. And even if something _thinks_ it is passing the latest run, due to airflow being a distributed system, something else could create a new run in the meantime, making it not the latest run anymore. For one thing, this behavior sometimes causes airflow to set the next_dagrun field back in time. We should probably at least rename this field. But it might also be worth looking at how it's called and possibly making some adjustments to the logic. ### Committer - [x] I acknowledge that I am a maintainer/committer of the Apache Airflow project.