Update Heroku sample strategies to heroku-22 stack

[SaschaSchwarze0]

Ubuntu 18 support is ending and Heroku already prevents their heroku-18 stack usage without a compatibility flag that we introduced in #1294.

We should update to heroku-22 and remove the compat flag.

[edmorley]

Hi! I noticed there are still some references to the EOL heroku/buildpacks:18 CNB builder image:
https://github.com/search?q=org%3Ashipwright-io+%22heroku%2Fbuildpacks%3A18%22&type=code

Whilst most are in tests, I see there are a couple of docs items that may need updating.

The canonical URLs for the Heroku CNB builder images are:
https://github.com/heroku/cnb-builder-images
https://hub.docker.com/r/heroku/builder