Update to Node 20

Node 16 is [now deprecated](https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/) and thus soon a deprecation warning will be displayed when this action runs. Please update to Node 20.

Related:

- https://github.com/zaproxy/action-api-scan/issues/29
- https://github.com/zaproxy/action-full-scan/issues/80