Bump org.jetbrains.kotlin.plugin.serialization from 1.9.20 to 1.9.21

Bumps [org.jetbrains.kotlin.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: org.jetbrains.kotlin.plugin.serialization
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

build.gradle.kts

@@ -3,7 +3,7 @@ import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
 
 plugins {
     kotlin("jvm") version "1.9.21"
-    id("org.jetbrains.kotlin.plugin.serialization") version "1.9.20"
+    id("org.jetbrains.kotlin.plugin.serialization") version "1.9.21"
     id("org.cqfn.diktat.diktat-gradle-plugin") version "1.2.5"
     `maven-publish`
 }