Bump actions/download-artifact from 4 to 8

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 4 to 8.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v4...v8)

---
updated-dependencies:
- dependency-name: actions/download-artifact
  dependency-version: '8'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/wheels.yml

@@ -397,7 +397,7 @@ jobs:
       name: testpypi
       url: https://test.pypi.org/p/imas-core
     steps:
-      - uses: actions/download-artifact@v4
+      - uses: actions/download-artifact@v8
         with:
           pattern: cibw-wheels-*
           merge-multiple: true  # avoid artifact subdirs below wheelhouse
@@ -424,7 +424,7 @@ jobs:
       name: pypi
       url: https://pypi.org/p/imas-core
     steps:
-      - uses: actions/download-artifact@v4
+      - uses: actions/download-artifact@v8
         with:
           pattern: cibw-wheels-*
           merge-multiple: true  # avoid artifact subdirs below wheelhouse