Upgrade Gradle to latest about vertx-examples

[vietj]

@jponge can you have a look at upgrading to latest Gradle versions ? currently there are issues due to too old version

MacBook-Pro-de-Julien:kotlintest julien$ ./gradlew 

FAILURE: Build failed with an exception.

* Where:
Build file '/Users/julien/java/vertx-examples/kotlin-examples/kotlintest/build.gradle.kts' line: 29

* What went wrong:
An exception occurred applying plugin request [id: 'org.jetbrains.kotlin.jvm', version: '1.4.10']
> Failed to apply plugin [id 'org.jetbrains.kotlin.jvm']
   > The current Gradle version 4.2.1 is not compatible with the Kotlin Gradle plugin. Please use Gradle 5.3 or newer, or the previous version of the Kotlin plugin.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

* Get more help at https://help.gradle.org