Update Greenland conversions following 2023a tzdb update

[valentinbonneaud]

Update the Greenland timezone conversion to use a timezone following the same offsets as Nuuk

Much of Greenland, represented by America/Nuuk, will continue to
observe DST using European Union rules.  When combined with
Greenland's decision not to change the clocks in fall 2023,
America/Nuuk therefore changes from -03/-02 to -02/-01 effective
2023-10-29 at 01:00 UTC.  (Thanks to Thomas M. Steenholdt.)
This change from 2022g doesn't affect timestamps until 2024-03-30,
and doesn't affect tm_isdst until 2023-03-25.