chore(deps): update plugin org.jetbrains.kotlin.jvm to v2.3.21

Author: renovate[bot]

packages/proguard-plugin/build.gradle.kts

@@ -1,5 +1,5 @@
 plugins {
-    kotlin("jvm") version "2.3.20"
+    kotlin("jvm") version "2.3.21"
     id("java-gradle-plugin")
     id("maven-publish")
 }