Bump actions/download-artifact from 5 to 6 (xarray-contrib#472)

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]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/testpypi-release.yaml

@@ -66,7 +66,7 @@ jobs:
           python-version: "3.13"
 
       - name: Get built artifact
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: releases
           path: dist