* Make Java libraries compatible with Gradle 7
* Make kotlin-spring compatible with Gradle 7
* Update samples to comply to Gradle 7.2
* Generate samples
* feat: add companion object to data classes if the implement the interface java.io.Serializable
* style: format template file
* style: adapt to project's original code style
* fix: add missing imports
* style: reverted mustache template to original styling
* test: generate additional samples for kotlin-springboot-reactive and kotlin-springboot
* docs: name full qualified classname of interface java.io.Serializable to prevent confusion with kotlinx.serialization