chore: codify subtle >=2.50 dependency

[kevaundray]

Even though we have:

subtle = { version = ">=2.5.0, <3.0" }

In our Cargo.toml, doing:

 cargo update -p subtle --precise 2.4.1

Will actually downgrade subtle to 2.4.1, because it is not being used anywhere explicitly in rust-eth-kzg.

Note, that it won't build but it doesn't stop us from doing the update and getting the less helpful erorr message.

[kevaundray]

The above seems to be a quirk with how workspaces work, so I'll put subtle in the bls12-381 crate only

[kevaundray]

I've opened up #237 which adds subtle in the bls12-381 crate.