date: make "iso-strict" conforming for the UTC timezone

date: make "iso-strict" conforming for the UTC timezone

ISO 8601-1:2020-12 specifies that a zero timezone offset must be denoted
with a "Z" suffix instead of the numeric "+00:00". Add the correponding
special case to show_date() and a new test.

Changing an established output format which might be depended on by
scripts is always problematic, but here we choose to adhere more closely
to the published standard.

Reported-by: Michael Osipov <michael.osipov@innomotics.com>
Signed-off-by: Beat Bolli <dev+git@drbeat.li>
Signed-off-by: Junio C Hamano <gitster@pobox.com>