Update upload-artifact action.

Author: hageboeck

.github/workflows/build-slides.yml

@@ -34,7 +34,7 @@ jobs:
           working_directory: talk
           extra_system_packages: "py-pygments"
       - name: Upload PDF as artifact
-        uses: actions/upload-artifact@v2
+        uses: actions/upload-artifact@v3
         with:
           name: PDF_${{matrix.version}}
           path: |