Bump org.jetbrains.kotlin.plugin.serialization in /ktor-app

Bumps [org.jetbrains.kotlin.plugin.serialization](https://github.com/JetBrains/kotlin) from 1.8.10 to 1.9.20.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.8.10...v1.9.20)

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

Signed-off-by: dependabot[bot] <support@github.com>

ktor-app/build.gradle.kts

@@ -6,7 +6,7 @@ val postgres_version: String by project
 plugins {
   kotlin("jvm") version "1.8.10"
   id("io.ktor.plugin") version "2.2.4"
-  id("org.jetbrains.kotlin.plugin.serialization") version "1.8.10"
+  id("org.jetbrains.kotlin.plugin.serialization") version "1.9.20"
 }
 
 group = "org.rogervinas"