[ical4j 4.x] Update `CalendarUidSplitter`

[rfc2822]

Update CalendarUidSplitter so that it uses icalj 4.x instead of 3.x. No functional changes.

In ical4j 4.x, there's already Calendar.split(), which we could maybe use to simplify our own code: https://github.com/ical4j/ical4j/blob/5ac389f4c0de7bd2af4503db28c1d33705201bb7/src/main/java/net/fortuna/ical4j/model/Calendar.java#L358