Fix Heroku deploy button

[Matho]

Hi @parndt

I have fixed the Heroku deploy button. This fixes open issues at #29 and refinery/refinerycms#3508

The first problem was, that ruby-2.6.5 is no longer available on heroku-20 and heroku-22 Heroku stacks. So I have set the default stack to "heroku-18" Note from heroku deploy proces:

This app is using the Heroku-18 stack, which is deprecated:
https://devcenter.heroku.com/changelog-items/2432
From April 30th 2023, Heroku-18 will be end-of-life, and apps using it
will no longer receive security updates, and be run at your own risk.
From May 1st 2023, builds will no longer be allowed for Heroku-18 apps.

The second issue was, that on 16 December 2021, Heroku changed the Node version to 16.13.1 (which was previously 12.16.2).
To fix this, I inserted 'buildpacks' section to app.json configuration and exact version of Node to package.json. Now the Node v12.13.0 is installed and is installed before installation of Ruby gems.

This fixes the Heroku deployment

Because I have added the "node": "12.13.0" to the package.json file, it raises error on localhost installations. The error is error app_name@0.1.0: The engine "node" is incompatible with this module. Expected version "12.13.0". Got "10.19.0" The 10.19.0 number is the version of Node you have installed on localhost. To ignore this warning and run the Node you have installed, simly ignore it with yarn install --ignore-engines I have added the info to the Readme.

Thanks,
Martin