Number of notification pushers on homeserver grows unbounded

#### Describe the bug.
While looking at my Synapse server's metrics I noticed that the number of Element iOS pushers grows significantly even while the number of Element iOS instances used by people on my homeserver isn't changing. This could be a contributing factor to the other notification bugs people have reported having with Element iOS.

This may be a Synapse bug but I am assuming it is an Element iOS bug because neither Element Android or Fluffychat display this behavior.

#### Expected behavior
Presumably there should only be one pusher for each instance of the app

#### Screenshots

When Synapse restarts it drops back to the correct number of pushers:

#### Contextual information:
- OS: iOS 14.7.1
- App Version: 1.4.7