Merge pull request optuna#5744 from nzw0301/update-actions/upload-artifact

Update sphinx build's `upload-artifact` version

Author: nabenabe0928

.github/workflows/sphinx-build.yml

@@ -56,13 +56,13 @@ jobs:
         make html
         cd ../
 
-    - uses: actions/upload-artifact@v3
+    - uses: actions/upload-artifact@v4
       with:
         name: built-html
         path: |
             docs/build/html
 
-    - uses: actions/upload-artifact@v3
+    - uses: actions/upload-artifact@v4
       with:
         name: tutorial
         path: |