Bump plugin.serialization from 2.0.20 to 2.0.21

Bumps [plugin.serialization](https://github.com/JetBrains/kotlin) from 2.0.20 to 2.0.21.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v2.0.21/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.0.20...v2.0.21)

---
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

@@ -3,7 +3,7 @@ plugins {
 
     id("com.google.devtools.ksp") version "2.0.20-1.0.25"
 
-    kotlin("plugin.serialization") version "2.0.20"
+    kotlin("plugin.serialization") version "2.0.21"
 
     // https://foso.github.io/Ktorfit/
     id("de.jensklingenberg.ktorfit") version "2.1.0"