Update app screenshot to Nextcloud 25 design

### Is your feature request related to a problem? Please describe.

https://raw.githubusercontent.com/nextcloud/screenshots/master/apps/Calendar/view_week.png shows the old design

### Describe the solution you'd like

Update
- [x] README
- [x] info.xml (app store)

### Describe alternatives you've considered

_No response_

### Additional context

_No response_