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

.github/workflows/packaging.yml

@@ -122,7 +122,7 @@ jobs:
       - name: Upload Installer
         id: unsigned-artifact
         if: github.repository == 'openra/openra'
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           path: build/windows/*.exe