cmd/go: assume Go 1.16 instead of Go 1.11 for dependencies that lack explicit 'go' directives

Fixes #45109
Updates #44976
Updates #36876

Change-Id: Icb00f8b6e0d4e076d82da1697e7058b9e7603916
Reviewed-on: https://go-review.googlesource.com/c/go/+/303229


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>