Migrate to the task processing API

The Text Processing API is deprecated and replaced by the Task Processing API since Nextcloud 30.

The Text Processing one will stay a few more NC major version. But the providers will progressively by migrated to the new API.

There is an equivalent in the Task Processing API for each task type you support in the Android client.

What needs to be adjusted to migrate:
* the endpoints to schedule a task, cancel a task, get the task history, get the available task types etc...
* the input parameter given to the schedule endpoint
* the parsing of the task object returned by the endpoints (the output structure has slightly changed)

The (shipped) "testing" app contains providers for the Task Processing API.

More details there: https://github.com/nextcloud/assistant/issues/114