Investigation Required: Impact of FCM Subscription API Removal

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The current library calls https://fcm.googleapis.com/fcm/connect/subscribe for the final FCM subscription step. Confirmed with the Firebase team, this URL is scheduled for removal on June 20, 2024. While this URL is not explicitly listed among the affected URLs, it is considered part of the "Legacy Web SDK".

The push-receiver library also appears to use this call.
Once the URL is removed, FCM subscriptions through push-receiver may no longer function correctly.

1. Investigate alternative libraries or methods for FCM subscriptions that do not rely on the deprecated URL.
2. Update existing code to use the alternative library, if necessary.
3. Prepare to transition to a new API as recommended by the Firebase team.

Reference Document

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