Revert "Run CI on 1.56.1 until next hashbrown release"

This reverts commit b6aa495.

CI should be find on 1.56.0 as of hashbrown 0.12.2.

Author: cuviper

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - rust: 1.56.1 # MSRV
+          - rust: 1.56.0 # MSRV
             features:
           - rust: stable
             features: serde
@@ -55,7 +55,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - rust: 1.56.1
+          - rust: 1.56.0
             target: thumbv6m-none-eabi
           - rust: stable
             target: thumbv6m-none-eabi