internal/history: document Go 1.15.6 and Go 1.14.13
internal/history: document Go 1.15.6 and Go 1.14.13

Updates golang/go#42973
Updates golang/go#42974

Change-Id: I3c6f12238e7b36a079d399f45c0974512badf6a4
Reviewed-on: https://go-review.googlesource.com/c/website/+/275116
Trust: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
Run-TryBot: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website