Replace `#[zeroize(drop)]` with `#[derive(ZeroizeOnDrop)]` when moving to `zeroize` 1.5.x

[dkasak]

In zeroize 1.5.x, the #[zeroize(drop)] attribute was deprecated and replaced with ZeroizeOnDrop. We can't switch to newer zeroize versions due to dalek, but once we do, we should replace all uses of #[zeroize(drop)].