Update org.jetbrains.kotlin to 1.6.21

Closes gh-11250

gradle.properties

@@ -4,7 +4,7 @@ springBootVersion=2.4.2
 springFrameworkVersion=6.0.0-SNAPSHOT
 openSamlVersion=3.4.6
 version=6.0.0-SNAPSHOT
-kotlinVersion=1.6.20-M1
+kotlinVersion=1.6.21
 samplesBranch=main
 org.gradle.jvmargs=-Xmx3g -XX:+HeapDumpOnOutOfMemoryError
 org.gradle.parallel=true