Adapt upcoming cards dashboard widget for mobile clients

[AndyScherzinger]

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

With nextcloud/server#33658 mobile clients can access app widget data.

Describe the solution you'd like

Adapt the upcoming cards widget to nextcloud/server#33658 once it's done.

Describe alternatives you've considered

No response

Additional context

No response

[julien-nc]

It's done with the current state of nextcloud/server#33658
I will adjust if there are some changes there.