Overhaul script plugins as convention plugins

[bric3]

This issue tracks migration/refactor progress script plugins to convention plugins.

⚠️ At this time the work is barely started, so the list of PRs or tasks here is incomplete.

• Introduce module-kind convention plugins #11620
• Remove unused Spring Boot Gradle script #12256
• Migrate SLF4J Simple test setup to a convention plugin #12257
• Migrate Scala test setup to a convention plugin #12260
• Migrate Kotlin test setup to a convention plugin #12284

Stack #12268