Bump actions/upload-artifact from 4 to 6 (#116)

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>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/typeset.yml

@@ -34,7 +34,7 @@ jobs:
           extra_system_packages: "graphviz"
           latexmk_shell_escape: true
       - name: Upload PDF file
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: PDF
           path: ledgersmb-book.pdf
@@ -68,12 +68,12 @@ jobs:
       - name: Build site
         run: ./rebuild-site.sh
       - name: Upload split page book site
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: split-book
           path: public_html/split-book
       - name: Upload single page book site
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: full-book
           path: public_html/full-book