ci: add Ruby 4.0 to test matrix

Author: rkoster

.github/workflows/ruby.yml

@@ -6,7 +6,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest]
-        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]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v6