title: Firebase Cloud Messaging Unsubscribe from all topics
body: <p>I have a simple news app for my college. To send notifications, I use firebase cloud messaging</p>
  <p>My problem: I want to send notifications based on the version of my app from the API. when my app version is 2. I will send a notification to the topic v2. but when users update the app to v3 and I need to maintain v2 of my app then how can I target v3 only</p>