Update org.jetbrains.kotlin to 1.8.20-RC

Closes gh-12899

gradle.properties

@@ -6,7 +6,7 @@ springFrameworkVersion=6.0.0
 micrometerVersion=1.10.5
 openSamlVersion=4.1.1
 version=6.1.0-SNAPSHOT
-kotlinVersion=1.8.10
+kotlinVersion=1.8.20-RC
 samplesBranch=main
 org.gradle.jvmargs=-Xmx3g -XX:+HeapDumpOnOutOfMemoryError
 org.gradle.parallel=true