Upgrade kotlin project generation to not mention kotlin.version with Gradle

[snicoll]

See spring-projects/spring-boot#11711 (comment)

Doing it now means we'd have three ways of generating a Kotlin-based project with Gradle:

1. In 1.x we do not have support for Kotlin in Spring Boot so everything must be generated
2. In 2.x up to 2.0.0.RC1 (not included) we can rely on Spring Boot's support to reduce the amount of build instructions but we have to set a kotlin.version
3. As of 2.0.0.RC1 we can but don't have to set the kotlin.version so we could just as well remove it

I am not fan of having three ways so we'll wait for 2.0 to be released to remove option 2.

[snicoll]

Looks like I misunderstood what needs to be done. According to @sdeleuze we're good with the current state.