ci: upgrade upload-pages-artifact to v4

actions/upload-pages-artifact@v3 uses Node.js 20 and will be deprecated
by GitHub in June 2026. Upgrade to v4.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: burningcost

.github/workflows/docs.yml

@@ -30,7 +30,7 @@ jobs:
       - name: Generate docs
         run: pdoc -o ./docs insurance_governance
       - name: Upload Pages artifact
-        uses: actions/upload-pages-artifact@v3
+        uses: actions/upload-pages-artifact@v4
         with:
           path: ./docs