Run CI on 1.56.1 until next hashbrown release

The PR changing hashbrown's `rust-version` from 1.56.1 to 1.56.0 has
been merged to master but not made it into a hashbrown release yet.

(cherry picked from commit b6aa495)

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - rust: 1.56.0 # MSRV
+          - rust: 1.56.1 # MSRV
             features:
           - rust: stable
             features: serde
@@ -59,7 +59,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - rust: 1.56.0
+          - rust: 1.56.1
             target: thumbv6m-none-eabi
           - rust: stable
             target: thumbv6m-none-eabi