crypto/internal/randutil: use math/rand/v2

crypto/internal/randutil: use math/rand/v2

Instead of using a select on a closed channel to generate one bit
of randomness, use math/rand/v2.

Change-Id: I36d294f17b9f5b437ca50e1c95da97bbe632c0a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/616475
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>