Ruby 3.1 is not available on the Windows CI environment

Author: shugo

.github/workflows/windows.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: windows-latest
     strategy:
       matrix:
-        ruby: [ 'mingw', 'mswin', '3.1', '2.7', '2.6' ]
+        ruby: [ 'mingw', 'mswin', '2.7', '2.6' ]
     steps:
     - uses: actions/checkout@v2
       with: