Bump actions/download-artifact from 4 to 6

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 4 to 6.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v4...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/pypi-publish.yaml

@@ -66,19 +66,19 @@ jobs:
     permissions:
       id-token: write
     steps:
-      - uses: actions/download-artifact@v4
+      - uses: actions/download-artifact@v6
         with:
           name: ubuntu-20.04-wheels
           path: artifacts/linux
-      - uses: actions/download-artifact@v4
+      - uses: actions/download-artifact@v6
         with:
           name: windows-2019-wheels
           path: artifacts/windows
-      - uses: actions/download-artifact@v4
+      - uses: actions/download-artifact@v6
         with:
           name: macos-latest-wheels
           path: artifacts/macos
-      - uses: actions/download-artifact@v4
+      - uses: actions/download-artifact@v6
         with:
           name: source-dist
           path: artifacts/sdist