The upgrade scripts are only for datamodel changes. I just basically did a diff between a 4.5 checkout and a 4.6 checkout. It looks like the cal_item_types table is new in 4.6, as is a new foreign key reference to that table in cal_items. The dates on those changes look like they are from March through July 2002.
It may be that calendar was not in a usable state in 4.5, so an upgrade script might not be absolutely needed if no-one is using a 4.5 version of calendar. I don't know though, so I left it alone 😊
