[release-branch.go1.21] go1.21.0

[release-branch.go1.21] go1.21.0

Change-Id: Iffac2ce43cd3a48ba420594adc3eab5aa9bcf113
Reviewed-on: https://go-review.googlesource.com/c/go/+/517055
Auto-Submit: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>