Bump actions/upload-artifact from 4 to 6

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 4 to 6.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v4...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] <support@github.com>

Author: dependabot[bot]

.github/workflows/build.yml

@@ -13,7 +13,7 @@ jobs:
           python-version: '3.10'
       - run: pip install jupytext
       - run: make notebooks
-      - uses: actions/upload-artifact@v4
+      - uses: actions/upload-artifact@v6
         with:
           name: notebooks
           path: |
@@ -30,7 +30,7 @@ jobs:
           python-version: '3.10'
       - run: pip install jinja2
       - run: make index
-      - uses: actions/upload-artifact@v4
+      - uses: actions/upload-artifact@v6
         with:
           name: index
           path: |
@@ -48,7 +48,7 @@ jobs:
           glob_root_file: true
           latexmk_shell_escape: true
           work_in_root_file_dir: true
-      - uses: actions/upload-artifact@v4
+      - uses: actions/upload-artifact@v6
         with:
           name: pdf
           path: "*/*.pdf"