Refactoring - `get_duration`, `get_due`, `get_dtend` to be obsoleted

[tobixen]

The duality between duration and dtend/due is IMO an annoying misfeature of the icalendar standard, so I made some helper methods for dealing with this - but it doesn't belong to the caldav library, it belongs to the icalendar library. Now we have the properties duration, dtend and dtstart available in the icalendar class. We should rewrite the current logic to wrap the icalendar properties and make some deprecation warnings.

[tobixen]

Claude suggests this is a 3-4 hours job - but it involves quite some rewriting, and I'm not sure it can be automated. I'd say 6 hours.

[tobixen]

Ref comments in the pull request, this has to wait not only for icalendar 7 to be released, but also for the icalendar 7 to be supported throughout the ecosystem. So it is a v4.0-task, not a v3.0-task.

Also, the current pull-request is deprecating and delegating the logic in the current methods, but that's just half the job - it's also needed to go through all internal calls and rewrite it.