Bump plugin.serialization from 1.9.22 to 2.1.21

Bumps [plugin.serialization](https://github.com/JetBrains/kotlin) from 1.9.22 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@v1.9.22...v2.1.21)

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

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

Author: dependabot[bot]

settings.gradle.kts

@@ -11,7 +11,7 @@ pluginManagement {
 
     plugins {
         kotlin("multiplatform") version "1.9.22"
-        kotlin("plugin.serialization") version "1.9.22"
+        kotlin("plugin.serialization") version "2.1.21"
     }
 }