chore(deps): update dependency apache-airflow-providers-slack to v4.1.0 (GoogleCloudPlatform#6794)

* chore(deps): update dependency apache-airflow-providers-slack to v4.1.0

* chore(deps): match requirement to constraint

* Update to Airflow 2.1.2, update constraints to match

Co-authored-by: Dan Lee <[email protected]>
Co-authored-by: Leah Cole <[email protected]>
Co-authored-by: Leah E. Cole <[email protected]>

Author: 3 people