Warn on EOL stacks (heroku-18, heroku-20)

[ipmb]

Summary

• Adds a warning log when an EOL stack (heroku-18 or heroku-20) is detected in app.json, directing users to upgrade to heroku-24
• Introduces EOLStacks slice to make it easy to extend as future stacks reach EOL
• Adds table-driven TestAppJsonBuilders covering heroku-18, heroku-20, heroku-22, and custom builders
• Adds TestAppJsonEOLStackWarning to verify the warning is emitted for all EOL stacks

Closes #25

Reference: https://help.heroku.com/NPN275RK/heroku-20-end-of-life-faq

Test plan

• cd builder && go test ./build/... passes
• Warning appears in build logs when app.json specifies heroku-20 or heroku-18

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