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/publish.yml

@@ -36,7 +36,7 @@ jobs:
           git commit -m "CI: render for school"
       # can be used for troubleshooting
       # - name: Store merged source
-      #   uses: actions/upload-artifact@v5
+      #   uses: actions/upload-artifact@v6
       #   with:
       #     name: ${{ matrix.school }}-src
       #     path: ./
@@ -52,7 +52,7 @@ jobs:
         run: git push origin ${{ matrix.school }}
         if: github.ref_name == 'main'
       - name: Store rendered HTML for link checking
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: ${{ matrix.school }}
           path: _build/html