Downgrade upload-pages-artifact action version from v4 to v3 in workflow

Author: tuna2134

.github/workflows/docs.yml

@@ -21,7 +21,7 @@ jobs:
           pdoc --html --config show_source_code=False --force voicevox
       
       - name: Upload artifact
-        uses: actions/upload-pages-artifact@v4
+        uses: actions/upload-pages-artifact@v3
         id: deployment
         with:
           path: ./html/voicevox