api: update next.txt

There's no reason not to, and it'll help me test an upcoming fix
for #43956. The API additions look reasonable to me, and they'll
go through a more comprehensive API audit during the freeze.

Change-Id: I0daa6e978b199d69568f5100fdfc1b4bcfaeaef2
Reviewed-on: https://go-review.googlesource.com/c/go/+/315349


Trust: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>