ci(deps): bump actions/download-artifact from 7 to 8

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 7 to 8.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v7...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/publish-nuget.yml

@@ -284,7 +284,7 @@ jobs:
 
       - name: Download package artifact
         id: download_nupkg
-        uses: actions/download-artifact@v7
+        uses: actions/download-artifact@v8
         with:
           name: dotnet-mcp-nupkg
           path: ./artifacts