ci: bump upload artifact version to 4

Author: tharropoulos

.github/workflows/tests.yml

@@ -37,7 +37,7 @@ jobs:
           bundler-cache: true # runs 'bundle install' and caches installed gems automatically
       - run: bundle exec rubocop
       - run: bundle exec rspec --format documentation
-      - uses: actions/upload-artifact@v3
+      - uses: actions/upload-artifact@v4
         with:
           name: coverage
           path: coverage/