doc: document removal of asynctimerchan GODEBUG setting (update release notes)

For #76472.

Change-Id: I1f470cee6b845e4d1fe4d48fd37ac11bbea26fda
Reviewed-on: https://go-review.googlesource.com/c/go/+/783421
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>