src: update go.mod files to go 1.21

src: update go.mod files to go 1.21

Go 1.21 is now in progress here.

For #36905.

Change-Id: Ib4d2271b3b8b5fe29e8358614a3d6d20a035e1a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/462199
Reviewed-by: Carlos Amedee <carlos@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>