doc/next: add linker change release notes

doc/next: add linker change release notes

Document that we now generate GNU build ID or Mach-O UUID by
default, and the related flags to disable or override it.

Fixes #68678.
Fixes #70586.
For #68545.

Change-Id: Ic79f3c05aab8a5032ba6983b8c72eeb4202bfe84
Reviewed-on: https://go-review.googlesource.com/c/go/+/633875
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>