fixup! Upgrade minimum required Ruby to v3.1.2

Author: acant

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
         ruby: ['3.1', '3.2', '3.4', head]
         test_task: ['tests']
         includes:
-          - ruby: 'head'
+          - ruby: head
             test_task: 'spec'
     continue-on-error: ${{ endsWith(matrix.ruby, 'head') || endsWith(matrix.ruby, 'jruby') }}
     runs-on: ${{ matrix.os }}