Migrate away from heroku-buildpack-static

[edmorley]

The heroku-buildpack-static buildpack is deprecated and will not be supported on Heroku-22 or newer stacks.

The create-react-app-buildpack buildpack currently uses the static buildpack, so will need to migrate to heroku-buildpack-nginx or else some other solution in order to (a) be compatible with Heroku-22 and later stacks, (b) so that it's using something maintained.

cc @mars

[mars]

I think it's time to sunset this buildpack, then 🌆 because moving from static.json to a new web server configuration format will be breaking for many existing users.

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