[build] Bump actions/download-artifact in generate-gh-pages: v1->v4

Author: AndreyAkinshin

.github/workflows/generate-gh-pages.yaml

@@ -48,7 +48,7 @@ jobs:
         ref: docs-stable
 
     - name: Download Artifacts
-      uses: actions/download-artifact@v1
+      uses: actions/download-artifact@v4
       with:
         name: site