Update required node version to v20

[peterwilsoncc]

Describe the bug

Since updating wp-env, an npm install is triggering a warning of an incompatible engine:

$ npm i
npm WARN EBADENGINE Unsupported engine {
npm WARN EBADENGINE   package: '@wordpress/env@10.5.0',
npm WARN EBADENGINE   required: { node: '>=18.12.0', npm: '>=8.19.2' },
npm WARN EBADENGINE   current: { node: 'v16.20.2', npm: '8.19.4' }
npm WARN EBADENGINE }

This can be resolved by updating the nvmrc file to version 20.

Steps to Reproduce

1. Checkout the develop branch
2. Run nvm use or equivalent to ensure the node version in use matches that defined in the config file
3. Run npm i
4. Observe the engine warning.

Screenshots, screen recording, code snippet

No response

Environment information

• macOS 14.6.1
• Distributor develop @ 14f4247

WordPress information

No response

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