internal/history: document upcoming Go 1.15.7 and 1.14.14 security releases
internal/history: document upcoming Go 1.15.7 and 1.14.14 security releases

They have been pre-announced¹.

¹ https://groups.google.com/g/golang-announce/c/KvrRblbXp_w

Change-Id: I6c55559a8598b8417f92bc1e8c69fd974270f462
Reviewed-on: https://go-review.googlesource.com/c/website/+/283536
Run-TryBot: Roland Shoemaker <0d270388f2f92757a5de0f4bd891d3b392c44c4f@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Trust: Roland Shoemaker <0d270388f2f92757a5de0f4bd891d3b392c44c4f@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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