Upgrade to Kotlin 1.4.21

See gh-26132

build.gradle

@@ -1,7 +1,7 @@
 plugins {
 	id 'io.spring.dependency-management' version '1.0.9.RELEASE' apply false
 	id 'io.spring.nohttp' version '0.0.5.RELEASE'
-	id 'org.jetbrains.kotlin.jvm' version '1.4.20' apply false
+	id 'org.jetbrains.kotlin.jvm' version '1.4.21' apply false
 	id 'org.jetbrains.dokka' version '0.10.1' apply false
 	id 'org.asciidoctor.jvm.convert' version '3.1.0'
 	id 'org.asciidoctor.jvm.pdf' version '3.1.0'
@@ -9,7 +9,7 @@ plugins {
 	id "io.freefair.aspectj" version '5.1.1' apply false
 	id "com.github.ben-manes.versions" version '0.28.0'
 	id "me.champeau.gradle.jmh" version "0.5.0" apply false
-	id "org.jetbrains.kotlin.plugin.serialization" version "1.4.20" apply false
+	id "org.jetbrains.kotlin.plugin.serialization" version "1.4.21" apply false
 }
 
 ext {
@@ -31,7 +31,7 @@ configure(allprojects) { project ->
 			mavenBom "io.r2dbc:r2dbc-bom:Arabba-SR8"
 			mavenBom "io.rsocket:rsocket-bom:1.1.0"
 			mavenBom "org.eclipse.jetty:jetty-bom:9.4.35.v20201120"
-			mavenBom "org.jetbrains.kotlin:kotlin-bom:1.4.20"
+			mavenBom "org.jetbrains.kotlin:kotlin-bom:1.4.21"
 			mavenBom "org.jetbrains.kotlinx:kotlinx-coroutines-bom:1.4.2"
 			mavenBom "org.junit:junit-bom:5.7.0"
 		}