Raise minimum tested toolchain to rust 1.56.1

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

@@ -20,7 +20,7 @@ jobs:
           - rust: beta
           - rust: stable
           - rust: 1.57.0
-          - rust: 1.48.0
+          - rust: 1.56.0
           - name: macOS
             rust: nightly
             os: macos
@@ -48,7 +48,7 @@ jobs:
         # builds.
         run: |
           echo RUSTFLAGS=$RUSTFLAGS >> $GITHUB_ENV
-          echo ::set-output name=exclude::--exclude cxx-test-suite ${{matrix.rust == '1.48.0' && '--exclude cxxbridge-cmd' || ''}}
+          echo ::set-output name=exclude::--exclude cxx-test-suite ${{matrix.rust == '1.56.0' && '--exclude cxxbridge-cmd' || ''}}
         env:
           RUSTFLAGS: ${{env.RUSTFLAGS}} ${{matrix.os && github.event_name != 'schedule' && '--cfg skip_ui_tests' || ''}}
         id: testsuite