Also update a Go 1 compatibility promise link to canonical URL.

Updates #27592
Updates #28264

Change-Id: I5994a0a63e0870c1795c65016590dfad829d26a7
Reviewed-on: https://go-review.googlesource.com/c/154618Reviewed-by: Richard Musiol <neelance@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>