lib/time, time/tzdata: update tzdata to 2020e

Changelog:

  Volgograd switches to Moscow time on 2020-12-27 at 02:00.
  Small changes to past timestamps and abbreviations.

See

  http://mm.icann.org/pipermail/tz-announce/2020-December/000063.html

Updates #22487

Change-Id: I709abe899ca498698463e945ccbcf4bc5fe60b92
Reviewed-on: https://go-review.googlesource.com/c/go/+/279794


Trust: Alberto Donizetti <alb.donizetti@gmail.com>
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>