Update kotlin monorepo to v2.1.21

Author: renovate[bot]

build.gradle

@@ -39,7 +39,7 @@ subprojects {
 		ext.maven_name='AtPlug test harness'
 		dependencies {
 			compileOnly "org.junit.jupiter:junit-jupiter-api:5.12.2"
-			implementation "org.jetbrains.kotlin:kotlin-reflect:2.1.0"
+			implementation "org.jetbrains.kotlin:kotlin-reflect:2.1.21"
 			implementation project(':atplug-runtime')
 		}
 	} else if (it.name == 'atplug-plugin-gradle') {
@@ -48,7 +48,7 @@ subprojects {
 		dependencies {
 			implementation "org.ow2.asm:asm:9.8"
 			implementation 'com.diffplug.gradle:goomph:4.3.0'
-			implementation 'org.jetbrains.kotlin:kotlin-gradle-plugin:2.1.0'
+			implementation 'org.jetbrains.kotlin:kotlin-gradle-plugin:2.1.21'
 			testImplementation 'org.assertj:assertj-core:3.27.3'
 		}
 		tasks.named('test') {