Auto-update NextJS version defined in project templates

[amclin]

Automate the process of tying the NextJS version used in project templates to the one defined in the generator's package.json

Right now, the generator gets updates on a regular basis from dependabot. But the generator doesn't use NextJS outside of unit testing, and updates to the generator don't get reflected in the templates used for the generated projects:
https://github.com/amclin/react-project-boilerplate/blob/master/templates/default.json#L11

The version defined in the generated project is explicit, because otherwise we can't ensure a minimum version is installed in the generated project. There have been bugs in the past where a generated project was autocreated using a version of NextJS older than the one the code actually required (mostly due to caching in build servers).

Automating or coupling between the generator and templates would prevent needing to manually manage updates like #307

Ideas on how this could be managed:

• Script a replacement variable that is populated with the generator's version when the generator is run
• Others?

Other considerations

Whatever solution should be reusable for other packages where we define explicit minimum package versions in the project templates.