Release 6.20.3: Extend display name field for external participants

[changerequest]

We already have an update task 'com.openexchange.groupware.update.tasks.CalendarE xtendDNColumnTask' but the corresponding table create script in 'calendar.sql' was not changed. So an additonal update task 'com.openexchange.groupware.update.tasks.CalendarE xtendDNColumnTaskV2' is necessary.