Push notifications don't work in f-droid distributed version

I'm creating this bug as the underlying issue behind several enhancement requests, because there are a few alternatives to solve this issue and it's hard to find all the related issues otherwise. Here is the list of related enhancement requests:

- #8684
- #5510
- #3333
- #1561

(maybe there are more I didn't find)

### Steps to reproduce
1. Download Nexcloud from https://f-droid.org/
2. Run it and wait for a notification to happen in the server (check via web)

### Expected behaviour
- The Nextcloud app should show a notification

### Actual behaviour
- No notification is shown by the Nextcloud app

### Can you reproduce this problem on https://try.nextcloud.com?
Yes.

### Environment data
Android version: 11

Device model: LG G6

Stock or customized system: LOS 18.1

Nextcloud app version: 3.16.1

Nextcloud server version: 21.0.3

Reverse proxy: no

### Logs
#### Web server error log
Not relevant

#### Nextcloud log (data/nextcloud.log)
Not relevant, this is a known issue and if you go to the Notifications screen, the app warns about this behavior.