Change suggested heroku stack images to the new url

[joshwlewis]

Summary

The image names / locations for the CNB-compatible Heroku stack images have changed. heroku/pack:* images still exist, but they won't be updated in the future. heroku/heroku:*-cnb(-build) are published alongside each new release of the heroku stack (heroku/heroku:*). We've done this to hopefully alleviate some of the confusion around our image names.

Output

Before

Stacks maintained by the community:

    Stack ID: heroku-20
    Description: The official Heroku stack based on Ubuntu 20.04
    Maintainer: Heroku
    Build Image: heroku/pack:20-build
    Run Image: heroku/pack:20

After

Stacks maintained by the community:

    Stack ID: heroku-20
    Description: The official Heroku stack based on Ubuntu 20.04
    Maintainer: Heroku
    Build Image: heroku/heroku:20-cnb-build
    Run Image: heroku/heroku:20-cnb

Documentation

• Should this change be documented?
  • No

Related

Introduction of new image tags: heroku/base-images#213

[jromero]

@joshwlewis thank you for the update. Do you mind signing the commit as required by our contributing guidelines?

[joshwlewis]

@jromero thanks for the review. Sorry I missed the signature (oops!). I've squashed and signed the commit. But that also disabled your auto-merge.