Enforcing the spring-boot-dependencies platform makes for too strong
an opinion about the version of Kotlin that should be on the build
script's classpath. It clashes with the version of Kotlin that's
embedded in Gradle and used with Gradle's Kotlin DSL.

This commit switches to a normal platform (rather than an enforced
platform) which allows it to express an opinion about the version of
Kotlin without making it a strict requirement.

Closes gh-19609