Bump actions/download-artifact from 6 to 8

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 6 to 8.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v6...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/stage-2-test.yaml

@@ -194,7 +194,7 @@ jobs:
         with:
           python-version: ${{ inputs.python_version }}
       - name: "Download all test coverage artefacts"
-        uses: actions/download-artifact@v6
+        uses: actions/download-artifact@v8
         with:
           path: gateway-api/test-artefacts/
           merge-multiple: false
@@ -224,7 +224,7 @@ jobs:
         with:
           fetch-depth: 0 # Full history is needed for better analysis
       - name: "Download merged coverage report"
-        uses: actions/download-artifact@v6
+        uses: actions/download-artifact@v8
         with:
           name: ${{ needs.create-coverage-name.outputs.coverage-name }}
           path: coverage-reports/