The `stack` property lived on the `buildpacks` plugin but was shared by both the herokuish and pack builders, so a value valid for one builder broke the other. Each builder now owns its own `stack` property managed via `builder-herokuish:set` and `builder-pack:set`, while `buildpacks:set-property stack` is deprecated and forwards the value to the builder detected from it; existing values migrate automatically on upgrade and changing a builder's stack clears that builder's cache.
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