Add Ruby 4.0 to tests workflow matrix

Author: tf

.github/workflows/tests.yml

@@ -27,6 +27,8 @@ jobs:
             gemfile: gemfiles/rails_6.x_active_admin_3.x.gemfile
           - ruby-version: '3.0'
             gemfile: gemfiles/rails_7.x_active_admin_3.x.gemfile
+          - ruby-version: '4.0'
+            gemfile: gemfiles/rails_7.x_active_admin_3.x.gemfile
 
     env:
       BUNDLE_GEMFILE: ${{ matrix.gemfile }}