iCalendar outputs do not handle UTC times properly

[jennifer-richards]

Describe the issue

Per RFC 5545, a DATE-TIME in an iCalendar file must represent UTC times using a Z suffix (e.g., 2022-08-08T23:01:00Z). For local times with a time zone reference, the TZID attribute is to be used (e.g., TZID=America/Los_Angeles:2022-08-08T16:01:00).

In at least agenda.ics, meeting time zones are specified using the TZID attribute regardless of its value. This should instead use the -Z suffix when the meeting times are in UTC.

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