Remove support for `heroku/buildpacks:20`

[edmorley]

It looks like this project still supports the heroku/buildpacks:20 builder image:

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

  Lines 34 to 54 in c1cf649

  	"heroku-20": {
  	// $ pack builder inspect heroku/buildpacks:20 -o json | jq '.remote_info.buildpacks[].id'
  	"heroku/builder-eol-warning",
  	"heroku/go",
  	"heroku/gradle",
  	"heroku/java",
  	"heroku/jvm",
  	"heroku/maven",
  	"heroku/nodejs",
  	"heroku/nodejs-corepack",
  	"heroku/nodejs-engine",
  	"heroku/nodejs-npm-engine",
  	"heroku/nodejs-npm-install",
  	"heroku/nodejs-pnpm-install",
  	"heroku/nodejs-yarn",
  	"heroku/php",
  	"heroku/procfile",
  	"heroku/python",
  	"heroku/ruby",
  	"heroku/scala",
  	},

• https://github.com/search?q=repo%3Aapppackio%2Fapppack-codebuild-image%20ALLOW_EOL_SHIMMED_BUILDER&type=code

That builder image hasn't received security updates since May 2024, and is due to be removed in the future:
heroku/cnb-builder-images#830

Ideally, I'd recommend dropping support for Heroku-20 entirely, since Ubuntu 20.04 is EOL.

Or failing that, either switch to heroku/builder:20 (which at least had updates until June 2025, though is still EOL/insecure), or switch to your own mirror of heroku/buildpacks:20 so you won't be affected by the future image removal.