Upgrade to Kotlin 1.5.21

Closes gh-27318

Author: snicoll

gradle.properties

@@ -4,7 +4,7 @@ org.gradle.caching=true
 org.gradle.parallel=true
 org.gradle.jvmargs=-Xmx2g -Dfile.encoding=UTF-8
 
-kotlinVersion=1.5.20
+kotlinVersion=1.5.21
 tomcatVersion=9.0.50
 
 kotlin.stdlib.default.dependency=false