Update org.jetbrains.kotlin to 1.8.21

Closes gh-13169

gradle.properties

@@ -6,7 +6,7 @@ springFrameworkVersion=6.0.8
 micrometerVersion=1.10.7
 openSamlVersion=4.1.1
 version=6.1.0-SNAPSHOT
-kotlinVersion=1.8.20
+kotlinVersion=1.8.21
 samplesBranch=main
 org.gradle.jvmargs=-Xmx3g -XX:+HeapDumpOnOutOfMemoryError
 org.gradle.parallel=true