ci: update msrv to 1.70.0

It feels like most crates moved on, so it's hard to keep up with older
versions.

Author: ctron

.github/workflows/ci.yaml

@@ -36,7 +36,7 @@ jobs:
           - windows-2019
         toolchain:
           - stable
-          - 1.56.0 # msrv (minimum supported rust version)
+          - 1.70.0 # msrv (minimum supported rust version)
 
     steps: