Add maintenance window UI for Deployment Managers

### Description

When running a large, multi team airflow deployment with multiple stakeholder teams and lots of dags, finding time to schedule routine maintenance can be difficult, and we have to rely on agreements from the teams that Airflow will not be running at agreed times e.g. Saturday at 11am-12pm.

### Use case/motivation

I would like a screen added to the UI that calculates periods where no dags are running, i.e. 'maintenance windows' so that deployment managers of Airflow can calculate when maintenance periods should be scheduled. This is also useful for other engineers to calculate when systems Airflow is dependent on should have 'safe' downtime.

### Related issues

_No response_

### Are you willing to submit a PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

The issue names no files, tests, or entry points. Start by locating the existing Airflow UI and DAG scheduling data, then clarify how maintenance windows should be calculated and displayed; done should be a UI screen that identifies periods when no DAGs are running.

Written by the indexing model from the issue text.