Default DTSTAMP has no timezone

[cweiske]

Events are required to have a DTSTAMP value, and that value must be in UTC:

Description: The value MUST be specified in the UTC time format.
https://datatracker.ietf.org/doc/html/rfc5545#section-3.8.7.2

With spatie/icalendar-generator 2.3.2 the value is in the current timezone, or without a timezone when withoutTimezone() is used:

DTSTAMP;TZID=Europe/Berlin:20220208T145035
DTSTAMP:20220208T144549

[cweiske]

See #78