Upgrade version of rand to 0.8

[giraffekey]

This crate uses version 0.6 of the rand crate and is incompatible with the latest 0.8

[tcharding]

This is because of the stated MSRV of 1.29, I assume once the other crates in the rust-bitcoin stack move to MSRV of 1.36 then this crate will also and rand can be updated. See rust-bitcoin/rust-secp256k1#328