Update ruby versions in CI (#923)

Author: nobu

.github/workflows/test.yml

@@ -6,12 +6,12 @@ jobs:
   build:
     strategy:
       matrix:
-        ruby: [ 'head', '3.1', '3.0', '2.7', '2.6' ]
+        ruby: [ 'head', '3.1', '3.0', '2.7' ]
         os:
           - windows-latest
           - ubuntu-latest
         include:
-          - { ruby: '2.7', os: 'macos-latest' }
+          - { ruby: '3.1', os: 'macos-latest' }
           - { ruby: 'jruby-9.2.19.0', os: 'ubuntu-latest' }
     runs-on: ${{ matrix.os }}
     steps: