Raise minimum tested toolchain to rust 1.56

Required by the transitive dev-dependency on once_cell.

    error: failed to parse manifest at github.com-1ecc6299db9ec823/once_cell-1.15.0/Cargo.toml

    Caused by:
      feature `edition2021` is required

      The package requires the Cargo feature called `edition2021`, but that feature is not stabilized in this version of Cargo (1.55.0 (32da73ab1 2021-08-23)).
      Consider trying a newer version of Cargo (this may require the nightly release).
      See https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#edition-2021 for more information about the status of this feature.

Author: dtolnay

.github/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [beta, stable, 1.46.0]
+        rust: [beta, stable, 1.56.0]
         include:
           - rust: nightly
             rustflags: --cfg async_trait_nightly_testing