chore(deps): bump actions/download-artifact from 4 to 8 (#9771)

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>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/backend_merge.yml

@@ -39,7 +39,7 @@ jobs:
       # (e.g. -cpu-vllm vs -cpu-vllm-omni). Must stay in sync with the
       # upload-artifact name in backend_build.yml.
       - name: Download digests
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v8
         with:
           pattern: digests${{ inputs.tag-suffix }}--*
           merge-multiple: true

.github/workflows/image_merge.yml

@@ -34,7 +34,7 @@ jobs:
       quay_username: ${{ secrets.quayUsername }}
     steps:
       - name: Download digests
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v8
         with:
           pattern: digests-localai${{ inputs.tag-suffix == '' && '-core' || inputs.tag-suffix }}-*
           merge-multiple: true