Support Node 20, address deprecation warnings

[nickynicolson]

This action now receives the following warning:

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: githubocto/flat@v3.4.0. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

(As per issue #76)

[theadventuresock]

Action seems to be completely unusable now without the update.

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: githubocto/flat@main. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.