Add Ruby 4.0 to the test matrix in GitHub Actions workflow (#379)

Author: olbrich

.github/workflows/tests.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ['3.2', '3.3', '3.4', 'jruby-10.0']
+        ruby: ['3.2', '3.3', '3.4', '4.0', 'jruby-10.0']
     env:
       BUNDLE_WITHOUT: optional
     steps: