Upgrade everything to eslint v7 - internal code and in the new app template

[flybayer]

What do you want and why?

eslint-config-react-app relatively recently added support for eslint v7 so we can upgrade all our internal tooling to to the latest eslint packages and we can upgrade all the eslint packages in the new blitz app template

I think this task should be fairly straightforward. Updating the major version numbers in the new app template will be manual, but all the other eslint packages in the repo should be able to be upgraded via the yarn upgrade-interactive --latest command.

[nettofarah]

hey, @flybayer. My friend @pragmaticivan and I wanted to try and tackle this issue together.
Here's pt1 blitz-js/blitz#852

Thanks!