maintner/maintnerd/maintapi: test x/website with Go 1.16 longer
maintner/maintnerd/maintapi: test x/website with Go 1.16 longer

Provide TryBot test coverage in x/website using Go 1.16
while that version is still used during its deployment.

For golang/go#51800.

Change-Id: I4b1361537a8af8322ec62c86891de46143758c4e
Reviewed-on: https://go-review.googlesource.com/c/build/+/393858
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Auto-Submit: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build