chore(deps): bump plugin.serialization from 2.1.20 to 2.1.21

Bumps [plugin.serialization](https://github.com/JetBrains/kotlin) from 2.1.20 to 2.1.21.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.1.20...v2.1.21)

---
updated-dependencies:
- dependency-name: plugin.serialization
  dependency-version: 2.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

build.gradle.kts

@@ -5,7 +5,7 @@ fun properties(key: String) = project.findProperty(key).toString()
 plugins {
     id("org.jetbrains.kotlin.jvm") version "2.1.21"
     id("org.jetbrains.intellij") version "1.17.4"
-    kotlin("plugin.serialization") version "2.1.20"
+    kotlin("plugin.serialization") version "2.1.21"
 
     // Gradle Changelog Plugin
     id("org.jetbrains.changelog") version "2.2.1"