build(deps): bump actions/download-artifact from 4 to 8

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 4 to 8.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](https://github.com/actions/download-artifact/compare/v4...v8)

---
updated-dependencies:
- dependency-name: actions/download-artifact
  dependency-version: '8'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

.github/workflows/snap-publish.yml

@@ -57,7 +57,7 @@ jobs:
       contents: read
     steps:
       - name: Download snap artifact
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v8
         with:
           name: etherpad-snap
 
@@ -79,7 +79,7 @@ jobs:
     environment: snap-store-stable
     steps:
       - name: Download snap artifact
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v8
         with:
           name: etherpad-snap