Add Ruby 4.0 to the test matrix

Author: djmb

.github/workflows/main.yml

@@ -27,7 +27,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby-version: ["3.2", "3.3", "3.4"]
+        ruby-version: ["3.2", "3.3", "3.4", "4.0"]
         gemfile: [ rails_7_2, rails_8_0, rails_8_1, rails_main ]
         database: [sqlite, postgres, mysql]
     services: