chore(core-deps): update plugin kotlin-serialization to v1.9.21

Author: renovate[bot]

gradle/libs.versions.toml

@@ -41,4 +41,4 @@ kotlin-testing = [
 dokka = { id ="org.jetbrains.dokka", version = "1.9.10" }
 kotlin-jvm = { id = "org.jetbrains.kotlin.jvm", version.ref = "kotlin" }
 kotlin-qa = { id = "org.danilopianini.gradle-kotlin-qa", version = "0.56.0" }
-kotlin-serialization = { id = "org.jetbrains.kotlin.plugin.serialization", version = "1.9.20" }
+kotlin-serialization = { id = "org.jetbrains.kotlin.plugin.serialization", version = "1.9.21" }