Add triggers to workflows based on members downgrading, being manually removed by an admin, etc

Currently, workflows can only trigger when a member joins a group. It would be very useful to also have a trigger that fires when a member is removed from a group (whether by canceling a subscription, being manually removed by an admin, or through an automated group rule). This would enable use cases like sending a follow-up message to canceled members, re-engagement sequences for members who lose access, and other post-cancellation automations — none of which are currently possible without manual intervention.