alternatecalendar: port `QDate` to `QCalendar::YearMonthDay`
Not all dates from some alternate calendar systems can be accommodated by QDate, for example, in Persian calendar, the second month has 31 days. QCalendar::YearMonthDay is a data structure to solve the problem.
alternateDateReady
is not changed since more changes will depend on some changes in KF6
BUG: 463196 FIXED-IN: 5.27