Refs https://github.com/Shopify/polaris-internal/issues/1524

[Node 16 is EOL](https://nodejs.org/en/about/previous-releases), and
[Node 18 is in Maintenance
Mode](https://nodejs.org/en/about/previous-releases), so we're updating
the minimum required Node version to v20.10.0 (the latest supported by
`node-releases`/`browserslist` at the time this project started).

## When is node used?

Node is used in 2 contexts:

1. By our consumers when they import, run, and bundle our libraries
2. By us, the Polaris team, while we develop our libraries

These are very different, and have their own configs within our
repository...

### 1. Consumer's node version

When a consumer uses one of our libraries, they're importing whatever is
in the `build/*` folder. Those assets are generated by our build process
(Rollup) which transforms (via babel) whatever version of Javascript we
write our code in into a version that's compatible with the [`targets`
setting we
provide](https://github.com/Shopify/polaris/blob/9c24a465c5e001e148bde335bf6319e924f4b1d6/polaris-react/rollup.config.mjs#L72).

For example, `Array#findLast` was only made available in node v18, so
when we write code like:

```javascript
[1, 2, 3, 2, 1].findLast(x => x < 2);
```

The output will be different depending on our Babel config:

```javascript
// with babel config: { targets: 'node 16.0' }
import "core-js/modules/esnext.array.find-last.js";
[1, 2, 3, 2, 1].findLast(x => x < 2);
```

```javascript
// with babel config: { targets: 'node 18.0' }
[1, 2, 3, 2, 1].findLast(x => x < 2);
```

_(try it
[here](https://babeljs.io/repl#?browsers=&build=&builtIns=usage&corejs=3.21&spec=false&loose=false&code_lz=NoRgNABATJDMkwiAugOgGYEsB2ATAMgIYDOALgBQAeEAvAHwTUA80AlANxA&debug=false&forceAllTransforms=false&modules=false&shippedProposals=false&circleciRepo=&evaluate=false&fileSize=false&timeTravel=false&sourceType=module&lineWrap=true&presets=env&prettier=false&targets=Node-16.0&version=7.24.0&externalPlugins=&assumptions=%7B%7D)
- set the "Env preset > Node" version to `18.0`)_

The `node 18.0` example above does not include a polyfill for
`Array#findLast`, which means if our consumers try to run Polaris in a
node environment (eg; during SSR) that doesn't support `Array#findLast`,
they will get an error.

The way we let our consumers know what is the minimum version of node
they can use is via the `packages.json#engines.node` field. As long as
this matches the version specified in `Babel#targets`, any consumer
trying to install our library in an unsupported version of Node will
receive an error.

This PR updates the minimum required node version of our consumers to
`20.10.0`.

### 2. Our node version while developing Polaris

Similarly, the tools we use to develop Polars (Babel, Postcss, etc) may
have a minimum node version requirement.

They do it by specifying a `package.json#engines.node` version in their
own library. Then when we install their package, if our version of node
isn't compatible we'll receive an error.

However, the version of node required to run Babel _has no impact on its
ability to transform our code to work on older versions of node_. As
long as we tell Babel which version of node to output code for in
`targets`, it doesn't matter what our `.nvmrc` file says.

#11739 updated the node version
for developing with Polaris to `20.11.1`.

---------

Co-authored-by: Aaron Casanova <[email protected]>