Bump oldest rustc for preserve_order feature 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.

Author: dtolnay

.github/workflows/ci.yml

@@ -33,7 +33,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [beta, stable, 1.53.0, 1.46.0, 1.45.0, 1.40.0, 1.38.0, 1.36.0]
+        rust: [beta, stable, 1.56.1, 1.53.0, 1.46.0, 1.45.0, 1.40.0, 1.38.0, 1.36.0]
         os: [ubuntu]
         include:
           - rust: stable
@@ -51,9 +51,9 @@ jobs:
       - run: cargo check --manifest-path tests/crate/Cargo.toml --no-default-features --features alloc
       - run: cargo check --manifest-path tests/crate/Cargo.toml --no-default-features --features alloc,raw_value
       - run: cargo check --features preserve_order
-        if: matrix.rust != '1.45.0' && matrix.rust != '1.40.0' && matrix.rust != '1.38.0' && matrix.rust != '1.36.0'
+        if: matrix.rust != '1.53.0' && matrix.rust != '1.46.0' && matrix.rust != '1.45.0' && matrix.rust != '1.40.0' && matrix.rust != '1.38.0' && matrix.rust != '1.36.0'
       - run: cargo check --manifest-path tests/crate/Cargo.toml --no-default-features --features alloc,preserve_order
-        if: matrix.rust != '1.45.0' && matrix.rust != '1.40.0' && matrix.rust != '1.38.0' && matrix.rust != '1.36.0'
+        if: matrix.rust != '1.53.0' && matrix.rust != '1.46.0' && matrix.rust != '1.45.0' && matrix.rust != '1.40.0' && matrix.rust != '1.38.0' && matrix.rust != '1.36.0'
       - name: Build without std
         run: |
           rustup target add aarch64-unknown-none