ci: bump upload-artifact→v7 and download-artifact→v8 for Node 24

The v5 line of these two actions still targets Node.js 20 (unlike
checkout/setup-node@v5, which are already on Node 24), so the deprecation
warning persisted. upload-artifact reaches Node 24 at v6+ and
download-artifact at v7+; move to the current latest (v7 / v8), which
share the same v4-generation artifact backend and pair correctly.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01DwbE3uTiFaKRNjx5vP2muV

.github/workflows/nsite-archive.yml

@@ -46,7 +46,7 @@ jobs:
       # `path: dist` stores the site files at the artifact root (index.html at top),
       # so the downloaded .zip extracts straight to the nsite site directory.
       - name: Upload site archive
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v7
         with:
           name: bitvid-site-${{ github.ref_name }}-${{ steps.vars.outputs.sha }}
           path: dist