ci: upgrade download-artifact from v4 to v8 (#733)

## Summary

Upgrade `actions/download-artifact` from v4 to v8 in the
`manual_regenerate_models` workflow.

The old v4 version relies on Node.js 20, which is approaching
end-of-life and produces deprecation warnings in CI runs. Upgrading to
v8 resolves these warnings and keeps the workflow on a supported
runtime.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>