Update plugin org.jetbrains.kotlin.jvm to v1.9.21

Author: renovate[bot]

Diff for: build.gradle

@@ -1,7 +1,7 @@
 plugins {
 	id "java-gradle-plugin"
 	id "groovy"
-	id "org.jetbrains.kotlin.jvm" version "1.9.10" // should match Kotlin version bundled with Gradle
+	id "org.jetbrains.kotlin.jvm" version "1.9.21" // should match Kotlin version bundled with Gradle
 	id "truststorebuilder-versioning"
 	id "truststorebuilder-docs"
 	id "truststorebuilder-publish"