contrib/tzdata: import tzdata 2021a
Merge commit '4cd7e1071de16a7392b0e466287f13e9e6f2081a'
Changes: https://github.com/eggert/tz/blob/2021a/NEWS
(cherry picked from commit 8c5bef2eb24cb191c87712a56a9860d8c29415a0)
(cherry picked from commit 09bdde595dd761fd3499b0b0eb085088b3d3276d)
Approved by: so