Update dependency nkeys 0.3.0 -> 0.3.1

The version 0.3.0 incorrectly uses ed25519-dalek.

To reproduce the problem:

    cargo +nightly update -Z minimal-versions
    cargo check --package async-nats --all-targets

Returned the following:

    error[E0599]: no associated item named `BYTE_SIZE` found for struct `ed25519_dalek::Signature` in the current scope
       --> /home/nepalez/.cargo/registry/src/index.crates.io-6f17d22bba15001f/nkeys-0.3.0/src/lib.rs:257:58
        |
    257 |         let mut fixedsig = [0; ed25519_dalek::Signature::BYTE_SIZE];
        |                                                          ^^^^^^^^^ associated item not found in `Signature`

    error[E0599]: no function or associated item named `from_bytes` found for struct `ed25519_dalek::Signature` in the current scope
       --> /home/nepalez/.cargo/registry/src/index.crates.io-6f17d22bba15001f/nkeys-0.3.0/src/lib.rs:259:47
        |
    259 |         let insig = ed25519_dalek::Signature::from_bytes(&fixedsig)?;
        |                                               ^^^^^^^^^^ function or associated item not found in `Signature`
        |
        = help: items from traits can only be used if the trait is in scope

async-nats/Cargo.toml

@@ -17,7 +17,7 @@ categories = ["network-programming", "api-bindings"]
 memchr = "2.4"
 bytes = { version = "1.4.0", features = ["serde"] }
 futures = { version = "0.3.28", default-features = false, features = ["std", "async-await"] }
-nkeys = "0.3.0"
+nkeys = "0.3.1"
 once_cell = "1.18.0"
 regex = "1.9.1"
 serde = { version = "1.0.184", features = ["derive"] }