[scheduler] Replace `event.start`, `event.end` and `rrule`'s dates with strings

[flaviendelangle]

In this PR:

• Change the format of SchedulerEvent['start'] to be string
• Change the format of SchedulerEvent['end'] to be string
• Change the format of RecurringEventRecurrenceRule['until'] to be string
• Update the docs

The string format should at least support ISO strings ('2026-01-28T11:31:29.081Z'), we can see later which other formats are important.