[sdk] set min-supported Node.js to ^18.19.0 || >=20.6.0

[trentm]

For the SDK 2.0 work we have bumped the min-supported node to v18, without being more explicit than this. In package.json files, we generally have:

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

Technically this means anything 18.0.0 or later is supported, though, of course, "engines" is advisory.

---

I propose that we set our min-supported Node.js versions to ^18.19.0 || >=20.6.0.

These are the versions (https://nodejs.org/en/blog/release/v18.19.0 released Nov 2023, https://nodejs.org/en/blog/release/v20.6.0 released Sep 2023) where the module.register() Node.js API was added. Having these as our base Node.js versions will vastly simplify our documentation for supporting zero-code instrumentation of ESM code -- once we are using module.register() (#4933) -- because it means we will no longer need to use --experimental-loader=@opentelemetry/instrumentation/hook.mjs.

With TypeScript, CommonJS, ESM, bundling, et al, we already have enough / too much complexity to explain to users for bootstrapping instrumentation.