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>

Author: vdusek

.github/workflows/manual_regenerate_models.yaml

@@ -78,7 +78,7 @@ jobs:
       # Skipped for manual runs — datamodel-codegen will fetch from the published spec URL instead.
       - name: Download OpenAPI spec artifact
         if: inputs.docs_workflow_run_id
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v8
         with:
           name: openapi-bundles
           path: openapi-spec