Fix LGSSM parallel filter likelihood - #455

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@8e5e7e5ab8b370d6c329ec480221332ada57f0ab, actions/setup-python@7f4fc3e22c37d6ff65e88745f38bd3157c663f7c. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/