[AIP-31] Update Airflow tutorial to use functional DAGs

[casassg]

Description

Change Airflow tutorial (or create new tutorial) using the new functional layer.

Use case / motivation

Help users onboard Airflow by reducing the complexity of the tutorial by adopting the new functional DAG definition layer.

Related Issues

Blocked by #8057

[kaxil]

@casassg @turbaszek Can one you create a Github issue for the "dag" decorator -- looks like that is the only other pieces missing for Airflow 2.0 with regards to Functional DAGs apart from other tickets mentioned in AIP-31 Task Flow API for nicer DAG definition

[mik-laj]

I think the guide should present a functional and functional approach. These are two different approaches and have different use cases. This is not a 1 to 1 alternative.

[casassg]

This specific issue is about the main tutorial. I think that one benefits from having a unique style (I prefer functional as its easier). We should add a guide for both styles though

[ashb]

Vikram has volunteered to tackle this