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 <71398022+dandhlee@users.noreply.github.com>
Co-authored-by: Leah Cole <coleleah@google.com>
Co-authored-by: Leah E. Cole <6719667+leahecole@users.noreply.github.com>