Suggest people search existing issues in error message

[schneems]

In our error messages, we suggest that people open an issue and do not provide a URL. Instead we should provide them with https://github.com/heroku/buildpacks-ruby/issues and suggest that people search for one. Leaving the suggestion that they open an issue implied.

The goal is to increase the signal to noise ratio and not accidentally spam the current buildpack maintainer.

Error message is here

buildpacks-ruby/buildpacks/ruby/src/user_errors.rs

Line 34 in b377e25

buildpack or the framework please open an issue on the buildpack's GitHub repository.

Additionally we should reference or link to the application contract https://github.com/heroku/buildpacks-ruby/blob/main/docs/application_contract.md