ExposureNotification iOS 14.2 Beta 2 to Beta 3 API Differences

ENCommon.h
Added #def ENStringify
Added #def ENStringifyExpansion
Modified ENDaysSinceOnsetOfSymptomsUnknown
AvailabilityDeprecation Message
FromAvailablenone
ToDeprecatedServer must provide keys with days_since_onset_of_symptoms set.