Require Node.js 14.19, 16.15 or 18

[novemberborn]

Node.js 12 is end of life.

Increase the minimal versions for Node.js 14 and 16 to get a more modern feature baseline we can rely on for future development during AVA 5's lifetime.

[sindresorhus]

In case it's useful, new stuff in Node.js 14:

• private class methods and accessors
• Top-level await
  • Get rid of all (async () => {})()
• crypto.randomInt (Node.js 14.10)
• Optional chaining
• Nullish Coalescing
• import fs from 'node:fs/promises';
• crypto.randomUUID() (Node.js 14.17)
• AbortController signal option (Node.js 14.18)

[novemberborn]

Lots of nice features we can make use of then!