README: update from CC-BY-3.0 to CC-BY-4.0

README: update from CC-BY-3.0 to CC-BY-4.0

CC-BY-3.0 was shiny and new back in 2009, but CC-BY-4.0 is
generally preferred now. Update our CC-BY uses to CC-BY-4.0.

Google lawyers signed off on the overall CC-BY-4.0 update
and Renee French signed off on the update of the gopher license.

See also CL 447156.

Change-Id: I3908910d6011ed733271e595f761c773351b30f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/447275
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>