Stop publishing non-functions images to the internal registry (#428)

The `heroku/buildpacks:20` and `heroku/builder:22` builders are now only
published to Docker Hub, rather than also being published to the
internal registry (where they were tagged as `heroku-20:builder` and
`heroku-22:builder` respectively).

Kodon has not defaulted to these non-functions images from the internal
registry for some time, and as of heroku/kodon#410 these images (and the
internal registry) are no longer on the builder allow list, so it's not
possible to manually select them using `project.toml`.

This change does not affect the functions-only image, which continues to
be published to (and consumed by the functions-specific Kodon instance
from) the internal registry.

GUS-W-14378185.

.github/workflows/build-test-publish.yml

@@ -146,14 +146,12 @@ jobs:
         include:
           - builder: buildpacks-20
             tag_public: heroku/buildpacks:20
-            tag_private: heroku-20:builder
           - builder: builder-classic-22
             tag_public: heroku/builder-classic:22
           - builder: builder-20
             tag_public: heroku/builder:20
           - builder: builder-22
             tag_public: heroku/builder:22
-            tag_private: heroku-22:builder
           - builder: salesforce-functions
             tag_private: heroku-22:builder-functions
     steps: