Bump kotlinVersion from 1.8.22 to 1.9.21

Bumps `kotlinVersion` from 1.8.22 to 1.9.21.

Updates `jvm` from 1.8.22 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@v1.8.22...v1.9.21)

Updates `plugin.serialization` from 1.8.22 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@v1.8.22...v1.9.21)

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

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

build.gradle.kts

@@ -1,5 +1,5 @@
 plugins {
-    val kotlinVersion = "1.8.22"
+    val kotlinVersion = "1.9.21"
     kotlin("jvm") version kotlinVersion
     kotlin("plugin.serialization") version kotlinVersion
     application