CI: Switch to Ruby 4.0

Signed-off-by: Tim Meusel <tim@bastelfreak.de>

Author: bastelfreak

.github/workflows/runtime_tests.yml

@@ -22,10 +22,10 @@ jobs:
         uses: actions/checkout@v6
         with:
           fetch-depth: 0
-      - name: Install Ruby version 3.3
+      - name: Install Ruby version 4.0
         uses: ruby/setup-ruby@v1
         with:
-          ruby-version: 3.3
+          ruby-version: 4.0
           bundler-cache: true
 
       - name: Vanagon inspect