Apply icalendar.el patch by Thomas Plass <thunk2@arcor.de>. Fix bug#34315.

Apply icalendar.el patch by Thomas Plass <thunk2@arcor.de>.  Fix bug#34315.

* lisp/calendar/icalendar.el (icalendar--convert-tz-offset): No DST
  when RDATE is present.
* lisp/calendar/icalendar.el (icalendar--parse-vtimezone): Use
  `icalendar--get-most-recent-observance'.
* (icalendar--get-most-recent-observance): New.
* (icalendar--decode-isodatetime): Add parameters source-zone, result-zone.
* (icalendar--decode-isoduration): Fix decoding days.
* test/lisp/calendar/icalendar-tests.el (icalendar--decode-isoduration):
  Add testcases.