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

For #76472.

Change-Id: I4d34fb7ebb34bf7d989e7a2c74349d1435ab5d36
Reviewed-on: https://go-review.googlesource.com/c/go/+/783420
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>