Disallow promoting when the buildpacks differ

### Required Terms

- [x] I agree to follow this project's [Code of Conduct](https://github.com/heroku/roadmap/blob/main/CODE_OF_CONDUCT.md)
- [x] I have read and accept the [Salesforce Program Agreement](https://www.salesforce.com/company/program-agreement/)

### What service(s) is this request for?

Dashboard, CLI

### Tell us about what you're trying to solve. What challenges are you facing?

When you promote an app, it reuses the slug.
So if the slug was built with a different buildpack configuration than what is configured for the environment you are promoting to, it should be disallowed.

Promoting the wrong buildpacks to production can lead to some nasty problems.

The user would then either need to adjust the buildpack configuration or deploy instead of promote.