Update org.jetbrains.kotlin to 1.5.21

Closes gh-10100

Author: eleftherias

gradle.properties

@@ -4,7 +4,7 @@ springBootVersion=2.4.2
 springFrameworkVersion=5.3.7
 openSamlVersion=3.4.6
 version=5.6.0-SNAPSHOT
-kotlinVersion=1.5.0
+kotlinVersion=1.5.21
 samplesBranch=main
 org.gradle.jvmargs=-Xmx3g -XX:MaxPermSize=2048m -XX:+HeapDumpOnOutOfMemoryError
 org.gradle.parallel=true