Drop Node.js 10

[lydell]

Closes #515

Node.js 10 became end-of-life 2021-04-30.

There is a pull request that needs worker_threads: https://github.com/rtfeldman/node-test-runner/pull/558.

The worker_threads module technically exists in Node.js 10, but is behind the --experimental-worker. In Node.js 12 the module is marked as stable and can be used without flags.

Since Node.js 10 has been end-of-life for many months now and we need newer features, it’s time to drop support for Node.js 10.

The new lowest supported version is Node.js 12.13.0, which is the oldest long term support release of Node.js 12: https://nodejs.org/es/blog/release/v12.13.0/

This PR:

• Updates the minium Node.js version to 12.13.0.
• Drops Node.js 10 from CI.
• Drops Node.js 15 from CI (since odd version numbers have short support, and Node.js 15 is end of life), and adds Node.js 16 (newest long term support) and Node.js 17 (current latest version).
• Resolves “we can do this instead when Node.js 10 is dropped” comments in the code.
• The above point replaced the rimraf dependency with native fs functions. However, Flow does not know of them. I tried updating Flow, but it didn’t help. So I had to add $FlowFixMe suppression comments there.

[harrysarson]

LGTM

[harrysarson]

closes #515

[harrysarson]

@lydell merge when ready :)