Bump oldest rustc tested to 1.56.1

Required by 2021 edition in hashbrown.

    error: failed to download `hashbrown v0.12.1`

    Caused by:
      unable to get packages from source

    Caused by:
      failed to parse manifest at github.com-1ecc6299db9ec823/hashbrown-0.12.1/Cargo.toml

    Caused by:
      failed to parse the `edition` key

    Caused by:
      this version of Cargo is older than the `2021` edition, and only supports `2015` and `2018` editions.

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [nightly, beta, stable, 1.46.0]
+        rust: [nightly, beta, stable, 1.56.1]
     steps:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@master