Bump actions/download-artifact from 4 to 6

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

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/image-publish.yml

@@ -421,7 +421,7 @@ jobs:
           echo "is_v_semverish=${is_v_semverish}" >> "$GITHUB_OUTPUT"
 
       # === DOWNLOAD ===
-      - uses: actions/download-artifact@v4
+      - uses: actions/download-artifact@v6
         with:
           pattern: platform-digests-*
           path: platform-digests
@@ -619,7 +619,7 @@ jobs:
           inputs.mode == 'BUILD' ||
           inputs.mode == 'CVMFS_BUILD' ||
           inputs.mode == 'CVMFS_REMOVE_THEN_BUILD'
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v6
         with:
           pattern: platform-digests-*
           path: platform-digests