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

@@ -45,7 +45,7 @@ jobs:
     runs-on: ubuntu-latest
     if: github.ref == 'refs/heads/main'
     steps:
-      - uses: actions/download-artifact@v5
+      - uses: actions/download-artifact@v6
         with:
           name: build
       - uses: peaceiris/actions-gh-pages@v4
@@ -69,7 +69,7 @@ jobs:
     needs: build
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/download-artifact@v5
+      - uses: actions/download-artifact@v6
         with:
           name: build
       - uses: actions/setup-node@v6
@@ -91,7 +91,7 @@ jobs:
     runs-on: ubuntu-latest
     if: github.ref == 'refs/heads/main'
     steps:
-      - uses: actions/download-artifact@v5
+      - uses: actions/download-artifact@v6
         with:
           name: build
       - uses: actions/setup-node@v6