ci: downgrade to .NET 9, update upload-pages-artifact to v4

Author: Jani Giannoudis

.github/workflows/pages.yml

@@ -47,7 +47,7 @@ jobs:
         run: cd docfx && docfx docfx.json
 
       - name: Upload Pages artifact
-        uses: actions/upload-pages-artifact@v3
+        uses: actions/upload-pages-artifact@v4
         with:
           path: docfx/_site