chore(deps): update plugin org.jetbrains.kotlin.plugin.serialization to v2.1.21 (#353)

This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| org.jetbrains.kotlin.plugin.serialization | `2.1.20` -> `2.1.21` |
[![age](https://developer.mend.io/api/mc/badges/age/maven/org.jetbrains.kotlin.plugin.serialization:org.jetbrains.kotlin.plugin.serialization.gradle.plugin/2.1.21?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![adoption](https://developer.mend.io/api/mc/badges/adoption/maven/org.jetbrains.kotlin.plugin.serialization:org.jetbrains.kotlin.plugin.serialization.gradle.plugin/2.1.21?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![passing](https://developer.mend.io/api/mc/badges/compatibility/maven/org.jetbrains.kotlin.plugin.serialization:org.jetbrains.kotlin.plugin.serialization.gradle.plugin/2.1.20/2.1.21?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![confidence](https://developer.mend.io/api/mc/badges/confidence/maven/org.jetbrains.kotlin.plugin.serialization:org.jetbrains.kotlin.plugin.serialization.gradle.plugin/2.1.20/2.1.21?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|

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build.gradle.kts

@@ -4,7 +4,7 @@ import org.jetbrains.kotlin.gradle.targets.js.yarn.yarn
 
 plugins {
     kotlin("multiplatform") version "2.1.21"
-    kotlin("plugin.serialization") version "2.1.20"
+    kotlin("plugin.serialization") version "2.1.21"
     id("io.kotest.multiplatform") version "6.0.0.M3"
     distribution
 }