Upgrade to Kotlin 1.3.20

Includes RxJava 2.2.6, Netty 4.1.33, Undertow 2.0.17.

Fixes #20945

build.gradle

@@ -11,7 +11,7 @@ buildscript {
 // 3rd party plugin repositories can be configured in settings.gradle
 plugins {
 	id "io.spring.dependency-management" version "1.0.5.RELEASE" apply false
-	id "org.jetbrains.kotlin.jvm" version "1.2.71" apply false
+	id "org.jetbrains.kotlin.jvm" version "1.3.20" apply false
 	id "org.jetbrains.dokka" version "0.9.17"
 	id "org.asciidoctor.convert" version "1.5.8"
 }
@@ -35,17 +35,17 @@ ext {
 	jackson2Version      = "2.9.8"
 	jettyVersion         = "9.4.14.v20181114"
 	junit5Version        = "5.3.2"
-	kotlinVersion        = "1.2.71"
+	kotlinVersion        = "1.3.20"
 	log4jVersion         = "2.11.1"
-	nettyVersion         = "4.1.32.Final"
+	nettyVersion         = "4.1.33.Final"
 	reactorVersion       = "Californium-SR4"
 	rxjavaVersion        = "1.3.8"
 	rxjavaAdapterVersion = "1.2.1"
-	rxjava2Version       = "2.2.5"
+	rxjava2Version       = "2.2.6"
 	slf4jVersion         = "1.7.25"	  // spring-jcl + consistent 3rd party deps
 	tiles3Version        = "3.0.8"
 	tomcatVersion        = "9.0.14"
-	undertowVersion      = "2.0.16.Final"
+	undertowVersion      = "2.0.17.Final"
 
 	gradleScriptDir = "${rootProject.projectDir}/gradle"
 	withoutJclOverSlf4J = {
@@ -117,8 +117,8 @@ configure(allprojects) { project ->
 		kotlinOptions {
 			jvmTarget = "1.8"
 			freeCompilerArgs = ["-Xjsr305=strict"]
-			apiVersion = "1.1"
-			languageVersion = "1.1"
+			apiVersion = "1.2"
+			languageVersion = "1.2"
 		}
 	}