Set max Node version 17.x.x

[zeroby0]

Heyo!

Builds currently fail with Node v18 and above on Netlify with this plugin (as well as with the https://github.com/zeroby0/netlify-plugin-11ty plugin I maintain).

Setting node version to <18.0.0 creates a warning and a better error message when building on versions 18 or above.

Example:

12:27:01 PM: npm WARN EBADENGINE Unsupported engine {
12:27:01 PM: npm WARN EBADENGINE   package: 'netlify-plugin-11ty@2.0.0-alpha',
12:27:01 PM: npm WARN EBADENGINE   required: { node: '>=12.18.0 <17.0.0' },
12:27:01 PM: npm WARN EBADENGINE   current: { node: 'v19.1.0', npm: '8.19.3' }
12:27:01 PM: npm WARN EBADENGINE }

12:27:30 PM: ──────────────────────────
12:27:30 PM:   Configuration error                                           
12:27:30 PM: ──────────────────────────
12:27:30 PM: ​
12:27:30 PM:   Error message
12:27:30 PM:   The Node.js version is 19.1.0 but the plugin "netlify-plugin-11ty" requires >=12.18.0 <17.0.0

Please also consider adding notes about failing builds in the readme. (example: https://github.com/zeroby0/netlify-plugin-11ty#my-builds-are-failing)

Thanks!!