Upgrade node version

From a recent CI run:

$ npm install --global pnpm
npm WARN EBADENGINE Unsupported engine {
npm WARN EBADENGINE   package: 'pnpm@8.1.0',
npm WARN EBADENGINE   required: { node: '>=16.14' },
npm WARN EBADENGINE   current: { node: 'v15.14.0', npm: '7.7.6' }
npm WARN EBADENGINE }
added 1 package, and audited 2 packages in 1s
1 package is looking for funding
  run `npm fund` for details
found 0 vulnerabilities
$ pnpm config set --location=project store-dir .pnpm-store
ERROR: This version of pnpm requires at least Node.js v16.14
The current version of Node.js is v15.14.0

We should indeed upgrade to 16 at least. I'm not sure why we use 15 at all, which is a development version.