crypto: use clear built-in

crypto: use clear built-in

Replace for loops with clear built-in, available since Go 1.21.

Change-Id: I16a2691a68042e9c5cd9bc4197690fa541a081eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/704877
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>