Tidy up DagFileProcessor logging (remove reload and double loading etc) by porting it to structlog

[ashb]

Include newsfragment about changed logging as Jed requested here #44972 (comment)

[amoghrajesh]

All logging was ported over to structlog via: #52651, and it was announced in release notes too for 3.1, closing this issue: https://airflow.apache.org/docs/apache-airflow/stable/release_notes.html#airflow-now-uses-structlog-everywhere