Update rand to 0.6

I changed the isaac random to always be the 32 bits one because the 64
bits version doesn't work with ReseedingRng as it requires the
generator's output to be u32 for some reasons.

The other main change is that the reseed method doesn't exist anymore
and now rand recreates a new generator when it needs to reseed so I had
to add a new method for when we don't want a generator to be reseeded.

Closes #23558

components/script/dom/crypto.rs

@@ -15,7 +15,7 @@ use js::jsapi::JSObject;
 use js::jsapi::Type;
 use js::rust::CustomAutoRooterGuard;
 use js::typedarray::ArrayBufferView;
-use servo_rand::{Rng, ServoRng};
+use servo_rand::{RngCore, ServoRng};
 use std::ptr::NonNull;
 
 unsafe_no_jsmanaged_fields!(ServoRng);