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/build-test-release.yml

@@ -84,7 +84,7 @@ jobs:
           python-version: ${{ matrix.python }}
 
       - name: Load built wheel file
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: power-grid-model-io
           path: wheelhouse/
@@ -115,7 +115,7 @@ jobs:
           python-version: ${{ matrix.python }}
 
       - name: Load built wheel file
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: power-grid-model-io
           path: wheelhouse/
@@ -141,7 +141,7 @@ jobs:
           python-version: "3.13"
 
       - name: Load built wheel file
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: power-grid-model-io
           path: wheelhouse/