Buildpack API 0.10: Stack Removal

[loewenstein]

Describe the Enhancement

Implement this CNB RFC to remove support for stack id buildpack uses API 0.10.

The primary change here is the environment variables that are available to the buildpack as well as the structure of the buildpack.toml.

There will also need to be a change to the stacks to add the information needed to set CNB_TARGET_ID, see the platform spec for more details.

Motivation

Having support for the latest API versions is important to support not only our own project use cases, but those of our users as well.

[dmikusa]

A couple of notes.

1. We can only implement this in v2 of libcnb. There are breaking changes required for this and I believe 0.8 or 0.9 (I think around extensions). I think this highlights the point that we need to get moving on a 2.0 release, but I was also waiting for 0.10 to be set so that we could include this.

2. Because we can't do this in 1.x, I'd like to clear out everything with respect to stacks in he 2.x branch, but I'm not sure if that's too aggressive. I'd like to get feedback on that point. Do folks think that we should continue to support stacks into v2? Does that seem necessary for transitional purposes? Or is it sufficient to say stay on 1.x until you're off stacks?