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](https://github.com/actions/upload-artifact/compare/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>

.github/workflows/build_iso.yml

@@ -121,7 +121,7 @@ jobs:
       - name: Upload ISO as artifact
         if: matrix.version != 'fake'
         id: upload
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: ${{ matrix.image_name }}-${{ matrix.version }}${{ matrix.flatpaks == 'false' && '' || format('-{0}', matrix.flatpaks) }}
           path: |