lib/time, time/tzdata: update tzdata to 2021a

lib/time, time/tzdata: update tzdata to 2021a

Changelog:

  South Sudan changes from +03 to +02 on 2021-02-01 at 00:00.

Release announcement:

  http://mm.icann.org/pipermail/tz-announce/2021-January/000065.html

Updates #22487

Change-Id: Ia0a1a7a8f5d47adac9782bc2a445f69e02440f77
Reviewed-on: https://go-review.googlesource.com/c/go/+/285719
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>