Allow Node.js LTS ‘Gallium’ (v16) release version

[colinrotherham]

This PR lowers the minimum Node.js 16 and npm versions allowed

Why? Across our projects we have a mismatch in our supported Node.js versions. This makes npm link a bit painful with tools like nvm and asdf as they partition dependencies by Node.js version

lts/gallium → https://github.com/alphagov/govuk-frontend/blob/main/.nvmrc
16.14.2 → https://github.com/alphagov/govuk-design-system/blob/main/.nvmrc
16.16.0 → https://github.com/alphagov/govuk-prototype-kit/blob/v13/.nvmrc

Before

Latest Node.js LTS ‘Gallium’ (v16) release when #2842 was opened

"engines": {
  "node": "^16.17.0",
  "npm": "^8.5.0"
},

After

All Node.js LTS ‘Gallium’ (v16) releases from initial LTS launch are now supported

"engines": {
  "node": "^16.13.0",
  "npm": "^8.1.0"
},

[36degrees]

Makes sense👍🏻

[colinrotherham]

Also opened:

alphagov/govuk-design-system#2399
alphagov/govuk-prototype-kit#1712

[colinrotherham]

Update now #2939, alphagov/govuk-prototype-kit#1712 and alphagov/govuk-design-system#2399 are merged:

lts/gallium → https://github.com/alphagov/govuk-frontend/blob/main/.nvmrc
lts/gallium → https://github.com/alphagov/govuk-design-system/blob/main/.nvmrc
lts/* → https://github.com/alphagov/govuk-prototype-kit/blob/v13/.nvmrc

Once development environments are upgraded, package.json engines field changed, GitHub workflows bumped up, tests pass (and Heroku stacks upgraded to match) etc we may also want the lts/* alias everywhere