Update plugin kotlin to v2.0.21 (#355)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

gradle/libs.versions.toml

@@ -6,7 +6,7 @@ coroutines-core = { module = "org.jetbrains.kotlinx:kotlinx-coroutines-core", ve
 coroutines-test = { module = "org.jetbrains.kotlinx:kotlinx-coroutines-test", version.ref = "coroutines" }
 
 [plugins]
-kotlin = { id = "org.jetbrains.kotlin.multiplatform", version = "2.0.20" }
+kotlin = { id = "org.jetbrains.kotlin.multiplatform", version = "2.0.21" }
 spotless = { id = "com.diffplug.spotless", version = "6.25.0" }
 publish = { id = "com.vanniktech.maven.publish", version = "0.29.0" }
 dokka = { id = "org.jetbrains.dokka", version = "1.9.20" }