Update url routes and naming

[bbovenzi]

If we integrate tree/graph/gantt/details views into a single page, then we will need to change how the url routes in airflow work.

The default url would change from /tree?dag_id=example_dag to /dags/example_dag and /dags/example_dag/gantt would then show the tree/gantt combined view.

We should also drop all tree nomenclature, because it is no longer a tree diagram. It is simply the default dag page.

Redirects should be used to keep backwards compatibility.

[potiuk]

+1

[bbovenzi]

Added in #20730