Bump plugin.serialization from 1.9.20 to 1.9.21

Bumps [plugin.serialization](https://github.com/JetBrains/kotlin) from 1.9.20 to 1.9.21.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.9.20...v1.9.21)

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

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

build.gradle.kts

@@ -9,7 +9,7 @@ plugins {
     id("com.diffplug.spotless") version "6.22.0"
     id("com.github.jakemarsden.git-hooks") version "0.0.2"
     id("io.github.platan.tests-execution-chart") version "0.6.0"
-    kotlin("plugin.serialization") version "1.9.20"
+    kotlin("plugin.serialization") version "1.9.21"
     id("pl.allegro.tech.build.axion-release") version "1.15.5"
     id("com.github.ben-manes.versions") version "0.50.0"
     id("io.github.gradle-nexus.publish-plugin") version "2.0.0-rc-1"

tests-execution-chart-commons/build.gradle.kts

@@ -1,6 +1,6 @@
 plugins {
     id("org.jetbrains.kotlin.jvm") version "1.9.21"
-    kotlin("plugin.serialization") version "1.9.20"
+    kotlin("plugin.serialization") version "1.9.21"
     `maven-publish`
     id("org.jetbrains.kotlinx.kover") version "0.7.4"
     signing