chore(deps): 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/books-build.yml

@@ -57,7 +57,7 @@ jobs:
           python scripts/forge_all.py --changed-only "${{ steps.base.outputs.base_sha }}"
 
       - name: Upload book artifacts
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: books-all
           path: |

.github/workflows/dropzone-build.yml

@@ -42,7 +42,7 @@ jobs:
         run: python scripts/bookforge.py
 
       - name: Upload book artifacts
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: bookforge-outputs
           path: |