ci: update actions/upload-artifact to v4

Author: G-Rath

.github/workflows/ci.yml

@@ -28,7 +28,7 @@ jobs:
           bundler-cache: true
       - run: COVERAGE=1 bundle exec rspec
 
-      - uses: actions/upload-artifact@v2
+      - uses: actions/upload-artifact@v4
         with:
           name: coverage-report
           path: 'coverage/*'