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] <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -220,7 +220,7 @@ jobs:
         use-sodium-vrf: false
 
     - name: Download benchmark executables
-      uses: actions/download-artifact@v5
+      uses: actions/download-artifact@v6
       with:
         name: benchmark-exes-${{ runner.os }}-${{ matrix.ghc }}
 
@@ -308,7 +308,7 @@ jobs:
           cp -r --no-preserve=mode,ownership result/ static/pdfs
 
       - name: Download haddocks
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: haddocks