use of deprecated associated function `from_slice`

When running:
```
$ RAYON_NUM_THREADS=8 cargo test -j 8
```

I get:
```
Compiling panetiere v0.1.0 (/home/agami/workspace/flashbots/panetiere)
warning: use of deprecated associated function `sha2::digest::generic_array::GenericArray::::from_slice`: please upgrade to generic-array 1.x
--> src/pke.rs:87:20
|
87 | Nonce::from_slice(&nonce),
| ^^^^^^^^^^
|
= note: `#[warn(deprecated)]` on by default
```

More details of my testing set up and script in https://collective.flashbots.net/t/testing-panetiere/5868

Start by running `RAYON_NUM_THREADS=8 cargo test -j 8` and inspect the deprecated `Nonce::from_slice` call at `src/pke.rs:87`. Check the surrounding nonce handling and dependency versions, then confirm the test run completes without this deprecation warning.

Written by the indexing model from the issue text.