Upgrade to Kotlin 1.3.21

See #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 "1.3.20" apply false
+	id "org.jetbrains.kotlin.jvm" version "1.3.21" apply false
 	id "org.jetbrains.dokka" version "0.9.17"
 	id "org.asciidoctor.convert" version "1.5.8"
 }
@@ -35,7 +35,7 @@ ext {
 	jackson2Version      = "2.9.8"
 	jettyVersion         = "9.4.14.v20181114"
 	junit5Version        = "5.4.0-RC2"
-	kotlinVersion        = "1.3.20"
+	kotlinVersion        = "1.3.21"
 	log4jVersion         = "2.11.1"
 	nettyVersion         = "4.1.33.Final"
 	reactorVersion       = "Californium-SR4"