Bump actions/upload-artifact from 4 to 6 (#61)

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -93,7 +93,7 @@ jobs:
         run: bin/rails db:test:prepare test test:system
 
       - name: Keep screenshots from failed system tests
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v7
         if: failure()
         with:
           name: screenshots