Clarify how long a car can be power cycled

[pawelrutkaq]

Currently, we use u32 for timestamp, which covers 49days ofthe car being power cycled. Consider if this is enough.

https://github.com/eclipse-score/lifecycle/pull/41/files#diff-f8e6acdb09ddc50ab5dc4a894f84f97e1adbe863d128cceb3a8945e7fdf18857