⬆️ Bump actions/upload-artifact from 5 to 6

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 5 to 6.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v5...v6)

---
updated-dependencies:
- dependency-name: actions/upload-artifact
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/cache.yml

@@ -24,7 +24,7 @@ jobs:
         run: |
           jb build lectures --path-output ./ -W --keep-going
       - name: Upload "_build" folder (cache)
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: build-cache
           path: _build

.github/workflows/ci.yml

@@ -60,7 +60,7 @@ jobs:
         run: |
           jb build -W --keep-going lectures --path-output ./
       - name: Save Build as Artifact
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: _build
           path: _build

.github/workflows/publish.yml

@@ -58,7 +58,7 @@ jobs:
         run: |
           jb build lectures --path-output ./ --builder=custom --custom-builder=jupyter
           zip -r download-notebooks.zip _build/jupyter
-      - uses: actions/upload-artifact@v5
+      - uses: actions/upload-artifact@v6
         with:
           name: download-notebooks
           path: download-notebooks.zip