[CI] Bump actions/download-artifact from 5 to 6

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 5 to 6.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v5...v6)

---
updated-dependencies:
- dependency-name: actions/download-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/documentation.yml

@@ -113,7 +113,7 @@ jobs:
         uses: actions/configure-pages@v5
 
       - name: 📦 Download documentation
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: haddocks-${{ runner.os }}-ghc-${{ env.ghc-version }}-cabal-${{ env.cabal-version }}
           path: haddocks