squash! Upgrade minimum required Ruby to v3.1.2

Try to exclude running the Cucumber/Aruba feature integration tests on
Ruby head. Neither Cucumbe or Aruba support Ruby head/3.5, excluding for
the moment.

.github/workflows/ci.yml

@@ -11,6 +11,9 @@ jobs:
         os: [ubuntu-latest, macos-latest]
         ruby: ['3.1', '3.2', '3.4', head]
         test_task: ['tests']
+      includes:
+        - ruby: 'head'
+          test_task: 'spec'
     continue-on-error: ${{ endsWith(matrix.ruby, 'head') || endsWith(matrix.ruby, 'jruby') }}
     runs-on: ${{ matrix.os }}
     steps: