Update tested Node.js versions to currently supported versions (18.x, 20.x)

[stevethedev]

What is the purpose of this pull request? (put an "X" next to item)

[ ] Documentation update [ ] Bug fix [ ] New feature [x] Other, please explain: Ensure automated tests are run using the currently-supported Node versions.

What changes did you make? (Give an overview)

1. Removed Node 12.x, 14.x, 16.x from tests because none of them are currently-supported.
2. Added Node 18.x, 20.x to tests because those are the currently-supported versions.

Which issue (if any) does this pull request address?

N/A

Is there anything you'd like reviewers to focus on?

No.

[welcome[bot]]

🙌 Thanks for opening this pull request! You're awesome.