🛠️ Repo: Updated tested Node versions to be LTS

[JoshuaKGoldberg]

Our current tested version range is 14, 16, and 18:

https://github.com/mochajs/mocha-examples/blob/d1d12ca27eb4b3d2da633c27cb367d2f3df9196b/.github/workflows/test.yml#L12-L21

...but:

• Per https://nodejs.org/en/about/previous-releases, only 18 is still in LTS

• Our typescript-tsx-esm-import example doesn't work in Node <18 (https://github.com/mochajs/mocha-examples/actions/runs/9796936227/job/27052448109)

  typescript-tsx-esm-import: > mocha --import=tsx
  typescript-tsx-esm-import: TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension ".ts" for /home/runner/work/mocha-examples/mocha-examples/packages/typescript-tsx-esm-import/src/index.spec.ts
  typescript-tsx-esm-import:     at new NodeError (node:internal/errors:387:5)
  typescript-tsx-esm-import:     at Object.getFileProtocolModuleFormat [as file:] (node:internal/modules/esm/get_format:75:11)
  ...

This isn't the main Mocha repository: we don't need complete exact parity with supported Node versions.

I'll bump our CI tests to be LTS versions: 18, 20, and 22.