Test against Ruby 3.3

Also upgrade checkout action to the latest version available

Author: tagliala

.github/workflows/ruby.yml

@@ -19,10 +19,10 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['2.6', '2.7', '3.0', '3.1', '3.2']
+        ruby-version: ['2.6', '2.7', '3.0', '3.1', '3.2', '3.3']
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Set up Ruby
     # To automatically get bug fixes and new Ruby versions for ruby/setup-ruby,
     # change this to (see https://github.com/ruby/setup-ruby#versioning):