Update default CNB builder image to `heroku/builder:24`

[edmorley]

We released heroku/builder:24, our new CNB builder image version based upon Ubuntu 24.04 LTS, in June:
heroku/buildpacks#12
https://github.com/heroku/cnb-builder-images#available-images

As of #6, this project's default CNB builder image is heroku/builder:22:

apppack-codebuild-image/builder/build/appjson.go

Line 30 in 12d8c01

const DefaultStack = "heroku-22"

It would be great to (a) add support for heroku/builder:24 in general, (b) update the default CNB builder image to heroku/builder:24 in good time, to avoid another rush when heroku/builder:22 eventually becomes EOL.

The heroku/builder:24 builder is now a multi-architecture image (AMD64 + ARM64), and offers smaller run image sizes thanks to dropping unnecessary build-time dependencies from the run image. The list of included buildpacks (and their versions) is identical between heroku/builder:22 and heroku/builder:24.

For more information on the differences between the two images, see:
https://devcenter.heroku.com/articles/heroku-24-stack#what-s-new
https://devcenter.heroku.com/articles/heroku-24-stack#upgrade-notes
https://devcenter.heroku.com/articles/stack-packages