Update org.jetbrains.kotlin to 1.6.20

Closes gh-11150

Author: rwinch

gradle.properties

@@ -4,7 +4,7 @@ springBootVersion=2.4.2
 springFrameworkVersion=5.3.17
 openSamlVersion=3.4.6
 version=5.7.0-SNAPSHOT
-kotlinVersion=1.6.20-RC
+kotlinVersion=1.6.20
 samplesBranch=5.7.x
 org.gradle.jvmargs=-Xmx3g -XX:MaxPermSize=2048m -XX:+HeapDumpOnOutOfMemoryError
 org.gradle.parallel=true