Go 1.16 is available on App Engine now, so use it.
Also, Go 1.17 has been released, meaning
we don't need to keep Go 1.15 builders working.
Delete all the Go 1.16 backports except the template packages,
which contain modifications we need.
Change-Id: I4612441a1484ce9272a22268da73d67f7624309b
Reviewed-on: https://go-review.googlesource.com/c/website/+/343935
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Jamal Carvalho <jamal@golang.org>