Use DateConverter from spezi-firebase-utils

[PaulGoldschmidt]

Problem

We currently rock our own implementation of DateConverter taken over from myHeartCounts - this leads us to storing the lastActiveDate and dateOfEnrollment with new Date() (easily en/decoded by object.toISOString())instead of admin.firestore.Timestamp.now()).

Solution

We should close the Issue #27 in spezi-firebase first and then implement the dateConverter from spezi-firebase-utils that shell use admin.firestore.Timestamp.now in the future here.

Additional context

This will be a pretty big update as we also need to think about the functions that are using the timestamps (e.g. the nudge creation)

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