Update minimum supported Node.js version to v18

[chrispcampbell]

The README says that minimum Node.js version is v14, and our GitHub Actions workflows have been running on v16 for a long time, but both have reached end of life.

We should update the docs to suggest using either v18 or v20 as the minimum. There's nothing in SDE yet that requires v20, but there was at least one place I saw this week where using v18 would be beneficial (to replace our use of the outdated byline package).

Since there might be some users who have reason to use an older version, I'd suggest using v18 as a minimum (and use that when building on GitHub Actions), and the README could recommend using v20 (latest LTS version).

This would also be a good time to add engines to the package.json for at least the cli package (but maybe someday we should add it for all packages that depend on Node):

{
  "engines": {
    "node": ">=18"
  }
}