api: promote next to go1.13

Change-Id: I394f942822a0db27418d3fbb25df6227ea199a5f
Reviewed-on: https://go-review.googlesource.com/c/go/+/183919
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>