Update plugin org.jetbrains.kotlin.jvm to v1.8.21

Author: renovate[bot]

build.gradle.kts

@@ -2,7 +2,7 @@ import org.ajoberstar.grgit.Grgit
 import java.util.*
 
 plugins {
-    kotlin("jvm") version "1.8.20"
+    kotlin("jvm") version "1.8.21"
     id("org.ajoberstar.grgit") version "5.2.0"
     `java-library`
     `maven-publish`

bukkit-extension/build.gradle.kts

@@ -2,7 +2,7 @@ import org.ajoberstar.grgit.Grgit
 import java.util.*
 
 plugins {
-    kotlin("jvm") version "1.8.20"
+    kotlin("jvm") version "1.8.21"
     `java-library`
     `maven-publish`
     signing