Bump actions/upload-artifact from 4 to 6

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 4 to 6.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](https://github.com/actions/upload-artifact/compare/v4...v6)

---
updated-dependencies:
- dependency-name: actions/upload-artifact
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/ci.yml

@@ -132,7 +132,7 @@ jobs:
 
       - name: Upload digest
         if: github.event_name == 'push'
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: digests-${{ strategy.job-index }}
           path: ${{ runner.temp }}/digests/*

compose.yml

@@ -85,7 +85,7 @@ services:
 
   # Rebuilds the CSS/JS bundles into pipeline/built_assets whenever the sources change
   assets:
-    image: node:25-slim
+    image: node:24-slim
     working_dir: /app
     volumes:
       - .:/app