Rename DAG (from models/dag) to SchedulerDAG -- or remove it!

### Body

It is easily confused with the DAG in task sdk.

Ideally we can remove. At least rename it.

comment from ash

> That should be renamed as SchedulerDAG really -- (or possibly it can be removed now) -- it's a) a compat import while that part isn't complete, and b) where scheduler specific DAG functions go

### Committer

- [x] I acknowledge that I am a maintainer/committer of the Apache Airflow project.

Start by tracing the models/dag module and its compatibility import relationship with the DAG in the task SDK. Determine whether the module can be removed; otherwise identify the references that require the SchedulerDAG name. Done means the ambiguity is eliminated through removal or a consistent rename without leaving incompatible references.

Written by the indexing model from the issue text.