Bump Kotlin version to keep integration test working

Author: smowton

java/ql/integration-tests/kotlin/all-platforms/gradle_groovy_app/app/build.gradle

@@ -8,7 +8,7 @@
 
 plugins {
     // Apply the org.jetbrains.kotlin.jvm Plugin to add support for Kotlin.
-    id 'org.jetbrains.kotlin.jvm' version '1.5.31'
+    id 'org.jetbrains.kotlin.jvm' version '1.6.0'
 
     // Apply the application plugin to add support for building a CLI application in Java.
     id 'application'