Drop ruby 3.1 + grape 3.2.1 matrix row (grape 3.2.1 requires Ruby >= 3.2)

Author: numbata

.github/workflows/ci.yml

@@ -35,7 +35,6 @@ jobs:
           - { ruby: '3.3', grape: '2.2.0' }
           - { ruby: '3.4', grape: '2.2.0' }
           - { ruby: 'head', grape: '2.2.0' }
-          - { ruby: '3.1', grape: '3.2.1' }
           - { ruby: '3.2', grape: '3.2.1' }
           - { ruby: '3.3', grape: '3.2.1' }
           - { ruby: '3.4', grape: '3.2.1' }