go1.26: remove crypto/rand.Reader argument from all keygen and signing calls

[aarongable]

As of go1.26, all the cryptography functions that we use will ignore their rand.Reader argument and use a secure internal source of randomness instead. We always just pass the default global rand.Reader, so we should be able to replace that with nil at all callsites.

Read more: golang/go#70942, https://antonz.org/go-1-26/#crypto-reader