Breaking: #77919 - Changed DateTime::ISO8601 to DateTime::ATOM

Description

The format DateTime::ISO8601 is not compatible with ISO-8601, but is left this way for backward compatibility reasons. The constant DateTime::ATOM or DATE_ATOM is used instead when rendering JsonViews via Extbase.

See https://php.net/manual/en/class.datetime.php#datetime.constants.iso8601 for more information