Calender Backup does not include shared calender

### ⚠️ Before posting ⚠️

- [X] This is a **bug**, not a question or an enhancement.
- [X] I've [searched for similar issues](https://github.com/nextcloud/android/issues) and didn't find a duplicate.
- [X] I've written a clear and descriptive title for this issue, not just "Bug" or "Crash".
- [X] I agree to follow Nextcloud's [Code of Conduct](https://nextcloud.com/contribute/code-of-conduct/).

### Steps to reproduce

1. Have a calender from your gmail account
2. Add a calender from another gmail account, that shared the calender with you
3. Use Contact and Calender Backup in the Nextcloud app
--> all calenders are backed up successfully, except the shared one

### Expected behaviour

Shared calender from another gmail account, that is integrated into my device's calender, is backed up as well.

### Actual behaviour

Shared calender is not included in the backups.

### Android version

14

### Device brand and model

OnePlus 8T

### Stock or custom OS?

Stock

### Nextcloud android app version

3.29.2

### Nextcloud server version

29

### Using a reverse proxy?

No

### Android logs

_No response_

### Server error logs

_No response_

### Additional information

_No response_