proposal: update to Node.js 20

[RadxaYuntian]

Checklist

• I am using the latest version of this action.
• I have read the latest README and followed the instructions.
• I have read the latest GitHub Actions official documentation and learned the basic spec and concepts.

Describe your proposal

Currently the following warning will be generated when using this action:

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: peaceiris/actions-mdbook@v1. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

Describe the solution you'd like

Update to Node.js 20.

Describe alternatives you've considered

Use something else.

Additional context

Biannual occurrence: #465

[peaceiris]

fixed by #500

[peaceiris]

The tag v2.0.0 has been released, and the tag v2 is also available, thanks!

https://github.com/peaceiris/actions-mdbook/releases/tag/v2.0.0