CI: Add ruby 4

Author: Yoann Lecuyer

.github/workflows/test.yml

@@ -10,7 +10,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [2.6, 2.7, '3.0', 3.1, 3.2, 3.3, 3.4, head]
+        ruby: [2.6, 2.7, '3.0', 3.1, 3.2, 3.3, 3.4, 4.0, head]
 
     steps:
     - name: Checkout repository