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/manual-epub.yml

@@ -22,7 +22,7 @@ jobs:
         run: ebook-convert user_guide/index.xhtml koreader_user_guide.epub
 
       - name: Upload
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: KOReader Manual EPUB
           path: koreader_user_guide.epub

.github/workflows/manual-pdf.yml

@@ -49,7 +49,7 @@ jobs:
           weasyprint --full-fonts "$f" "$out"
 
       - name: Upload
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: KOReader Manual PDF - ${{ matrix.lang }}
           path: koreader_user_guide_${{ matrix.lang }}.pdf