[ical4j 4.x] Adding `EXDATE` to recurring event using UTC

[cketti]

I created an event using the web interface at https://webmail.vivaldi.net. It used UTC for all DATE-TIME values:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Roundcube libcalendaring 1.6-git//Sabre//Sabre VObject 2.1.7//EN
CALSCALE:GREGORIAN
BEGIN:VEVENT
UID:9340D1798F7702D4704D5DF52606029F-BC54928E7B083564
DTSTAMP;VALUE=DATE-TIME:20260415T203138Z
DTSTART;VALUE=DATE-TIME:20260406T120000Z
DTEND;VALUE=DATE-TIME:20260406T130000Z
SUMMARY:Recurring event
RRULE:FREQ=WEEKLY;INTERVAL=1
TRANSP:OPAQUE
END:VEVENT
END:VCALENDAR

Removing one of the instances (adding EXDATE) on the device leads to DAVx5 creating the following iCalendar object:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:DAVx5/4.5.11-alpha.1-ose ical4j/4.2.5
BEGIN:VEVENT
DTSTAMP:20260415T203246Z
UID:9340D1798F7702D4704D5DF52606029F-BC54928E7B083564
SUMMARY:Recurring event
DTSTART;TZID=Z:20260406T120000Z
DTEND;TZID=Z:20260406T130000Z
RRULE:FREQ=WEEKLY;INTERVAL=1
EXDATE;TZID=UTC:20260413T120000
END:VEVENT
END:VCALENDAR

It probably should be:

…
DTSTART:20260406T120000Z
DTEND:20260406T130000Z
…
EXDATE:20260413T120000Z
…