CI: add Ruby 4.0 to the test matrix

Author: orien

.github/workflows/test.yml

@@ -8,7 +8,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ ubuntu ]
-        ruby: [ '2.4', '2.5', '2.6', '2.7', '3.0', '3.1', '3.2', '3.3', '3.4' ]
+        ruby: [ '2.4', '2.5', '2.6', '2.7', '3.0', '3.1', '3.2', '3.3', '3.4', '4.0' ]
         include:
           - os: macos
             ruby: '2.7'