[bug] `random.string` function does not use `rand_chacha::ChaCha20Rng` about realm

[flemingcaleb]

As mentioned on the MR, its my understanding of the documentation of StdRng here that it uses ChaCha20 internally and is cryptographically equivalent but with the added benefit of periodic reseeding. I'm happy to make the change if wanted, but I do think it's worth considering staying with the default configuration if it is equally secure. That way if there is a change in the future to address some issue we will automatically inherit the change once it hits the standard library instead of needing to make internal modifications.

[Cictrone]

I do think we want this change. ChaCha20Rng is a standard way to operate a CSPRNG and StdRng just does not make that guarantee/promise.