Recent change to undefined template variable behavior

[Logan Attwood]

Hi Airflow-dev,

A change to how Airflow/jinja2 handles undefined variables inside
templates was recently merged, and this will cause DAG
rendering/execution to fail when using an undefined variable. Given
the potential impact of the change, sending this email seemed to be
the decent thing to do, and Kamil Breguła suggested it as well. Change
details are available here:
https://github.com/apache/airflow/pull/11016

Instructions on how to use the previous (dangerous) behaviour are
detailed inside UPDATING.md.

Cheers,
Logan Attwood