Upgrade Kotlin version (#141)

Author: carlonzo

build.gradle

@@ -1,5 +1,5 @@
 buildscript {
-  ext.kotlinVersion = '1.3.50'
+  ext.kotlinVersion = '1.5.31'
 
   repositories {
     mavenCentral()
@@ -67,7 +67,6 @@ repositories {
 
 dependencies {
   implementation localGroovy()
-  implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlinVersion"
   implementation gradleApi()
   api "guru.nidi:graphviz-java:0.8.1"