[CI] Update the upload-artifact action because Node 20 is deprecated.

Author: hageboeck

.github/workflows/build-slides.yml

@@ -43,7 +43,7 @@ jobs:
           working_directory: talk
           extra_system_packages: "py-pygments"
       - name: Upload PDF as artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: PDF_${{matrix.version}}
           path: |