Use Detekt to perform static analysis of Kotlin code

[wilkinsona]

Inspired by #45555 and #45574, we could apply Detekt's Gradle plugin as part of our Kotlin conventions. https://github.com/wilkinsona/spring-boot/tree/detekt does so with configuration that only checks for unused imports. We could expand this if we want by enabling more rules. Detekt's defaults find many more problems.