Backup Contacts does not include categories

### ⚠️ 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. Create a contact with two contact groups "category1" and "cat2"
2. Enable backup of contacts
3. search for contacts vcard in a backup *.vcf file in `.Contacts-Backup`
4. The contacts vcard doesn't include contact groups

I use Thundebirds CardBook AddOn to sync with nextcloud. Exporting the contact gives me a line `CATEGORIES:category1, cat2`

Exporting the contact via Nextcloud Contacts Web App (download) includes the `CATEGORIES` line, too

### Expected behaviour

Contact groups should also be included when backing up contacts

### Actual behaviour

The vcard misses the `CATEGORIES:...` line

### Android version

16

### Device brand and model

Pixel 8

### Stock or custom OS?

Stock

### Nextcloud android app version

3.33.0.RC3

### Nextcloud server version

31.0.9

### Using a reverse proxy?

No

### Android logs

_No response_

### Server error logs

```bash

```

### Additional information

I think, creating a vcard entry while creating the backup does not log single fields that are exported.