Add testing with Ruby 3.4

It's the latest and is getting updates until 2028

Author: huwd

.github/workflows/test.yaml

@@ -14,7 +14,7 @@ jobs:
 
     strategy:
       matrix:
-        ruby: ['3.2', '3.3']
+        ruby: ['3.2', '3.3', '3.4']
 
     steps:
       - uses: actions/checkout@v4