fix: update upload-artifact action from v3 to v4

GitHub deprecated v3 of the artifact actions, causing workflow failures.
Updated to v4 to resolve the CI/CD pipeline issue.

Author: claffin

.github/workflows/main.yml

@@ -84,7 +84,7 @@ jobs:
 
       - name: 📸 Upload E2E test artifacts
         if: failure()
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: cypress-screenshots
           path: cloudproxy-ui/cypress/screenshots