Upgrade to Kotlin 1.4.21

Closes gh-24391

Author: snicoll

gradle.properties

@@ -4,5 +4,5 @@ org.gradle.caching=true
 org.gradle.parallel=true
 org.gradle.jvmargs=-Xmx2g -Dfile.encoding=UTF-8
 
-kotlinVersion=1.4.10
+kotlinVersion=1.4.21
 kotlin.stdlib.default.dependency=false