CI against Ruby 3.4

Ruby 3.4 has been released, and support for Ruby 3.4.0 and 3.4.1 was added to setup-ruby:
ruby/setup-ruby#679

.github/workflows/rubocop.yml

@@ -29,7 +29,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu]
-        ruby: [2.7, "3.0", 3.1, 3.2, 3.3, head]
+        ruby: [2.7, "3.0", 3.1, 3.2, 3.3, 3.4, head]
         coverage: [null]
         modern: [null]
         title: [null]
@@ -105,7 +105,7 @@ jobs:
         internal_investigation: [null]
         include:
           - { rubocop: master, ruby: "3.0", os: ubuntu }
-          - { rubocop: "v1.61.0", ruby: "3.3", os: ubuntu }
+          - { rubocop: "v1.61.0", ruby: "3.4", os: ubuntu }
     steps:
       - name: checkout
         uses: actions/checkout@v4