Use Ruby 3.1 for CI

Author: aycabta

.github/workflows/test.yml

@@ -6,7 +6,7 @@ jobs:
   build:
     strategy:
       matrix:
-        ruby: [ 'head', '3.0', '2.7', '2.6' ]
+        ruby: [ 'head', '3.1', '3.0', '2.7', '2.6' ]
         os:
           - windows-latest
           - ubuntu-latest