Adapt to Personio API changes

[lyind]

Summary

Personio removed several fields from their TimeOffType response object:

    legacy_category
    half_day_requests_enabled
    certification_required
    certification_submission_timeframe
    substitute_option

This PR removes all use of these fields in the library.

[pipo02mix]

but does it mean that half days still exist?

[lyind]

but does it mean that half days still exist?

Yes. In fact, this means everything is now tracked in half-days instead of days. This is a first step (Personio did that internally to work towards improvements).

The switch to hourly tracking is separate from this.