Bump actions/download-artifact from 5 to 6 (open-telemetry#587)

Author: dependabot[bot]

.github/workflows/nginx.yml

@@ -36,7 +36,7 @@ jobs:
         run: |
           mkdir artifacts
       - name: Download artifacts
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           path: artifacts
           merge-multiple: true