Switch LoggingMixin to return structlog.BoundLogger

### Body This is so that we can switch to structlog universally in the airflow codebase, which lets us use its features re binding vars and context vars. I started work on this in https://github.com/apache/airflow/pull/46897 but have to drop it for now. Anyone with time and interest could pick it up. May just need to fix the tests. ### Committer - [x] I acknowledge that I am a maintainer/committer of the Apache Airflow project.