Add Global Disable switch / kill switch / maintenance mode #73950

Description

Currently Dags can be enabled or disabled on a per-dag level, but the airflow deployment itself cannot be disabled. This means that if a new airflow deployment is brought up, and dags are automatically enabled (e.g. by setting is_paused_upon_creation=False) then they will start automatically running as soon as they are parsed and the instance is deployed.

Use case/motivation

Add a global toggle switch that can be managed by Deployment managers / (Super) Admins that disables all scheduling of dags when toggled.
This could be used to put airflow into a 'maintenance mode' where no tasks will be scheduled, but running tasks can be drained, and the UI is still accessible. This allows deployment managers to perform maintenance work such as database upgrades, airflow upgrades etc. with confidence that nothing will be scheduled until the toggle is re-enabled.

Related issues

#70172

Are you willing to submit a PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct