Update DagVersion instead of stamping new one if there are no DAG runs using that version

### Body We should only keep around DAG versions that have a corresponding run. I believe the easy answer is to allow DAGs to update in place, without stamping a new version, if there are no runs on that version. We should be careful of race conditions while we are at it. ### Committer - [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.