internal/imports: update stdlib index for Go 1.21.0

For golang/go#38706.

Change-Id: I278b5edd7caf6ddb0585a3ea8f47777753457ddf
Reviewed-on: https://go-review.googlesource.com/c/tools/+/517075
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Gopher Robot <gobot@golang.org>
Run-TryBot: Gopher Robot <gobot@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com>