Upgrade to Kotlin 1.6.0

Closes gh-28698

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=1.6.0-RC2
+kotlinVersion=1.6.0
 tomcatVersion=9.0.54
 
 kotlin.stdlib.default.dependency=false