ci: pin upload-artifact to v3 for Gitea compatibility

Gitea Actions rejects @actions/artifact v2.0+ (upload-artifact@v4,
download-artifact@v4) with a GHESNotSupportedError. v3 is the highest
supported on current Gitea releases.

.gitea/workflows/ci.yaml

@@ -84,7 +84,7 @@ jobs:
           TEST_USERNAME: ${{ secrets.E2E_USERNAME }}
           TEST_PASSWORD: ${{ secrets.E2E_PASSWORD }}
 
-      - uses: actions/upload-artifact@v4
+      - uses: actions/upload-artifact@v3
         if: always()
         with:
           name: playwright-report