[release-branch.go1.21] spec: update spec to version at tip
This updates the spec by copying over several recent CLs
describing the new type inference mechanisms.
Fixes #61659.
Change-Id: I750c901e73e0404f782a3632f5cd936e3775ae13
Reviewed-on: https://go-review.googlesource.com/c/go/+/514435 Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Bypass: Robert Griesemer <gri@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>