Migrate gradle build files to kotlin dsl

[atomfrede]

Overview of the feature request

Gradle has added the kotlin dsl already a few years ago. While the groovy dsl can still be used the default (docs, start.spring.io and others like micronaut) is the kotlin dsl.

Motivation for or Use Case

There is no eol for the groovy dsl, but to make sure we are not forced to change it we should do it now. Besides that the kotlin dsl has some nice effects when it comes to code completion etc.
With this we would be in line with jhipster lite, which have been using the kotlin dsl right from the beginning.

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