[Tech] bump plugin.serialization from 1.9.21 to 2.0.20 in /backend

Bumps [plugin.serialization](https://github.com/JetBrains/kotlin) from 1.9.21 to 2.0.20.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.9.21...v2.0.20)

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

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

backend/build.gradle.kts

@@ -8,7 +8,7 @@ plugins {
     kotlin("plugin.noarg") version "1.9.10"
     kotlin("plugin.jpa") version "2.0.0"
     id("org.jlleitschuh.gradle.ktlint") version "11.6.1"
-    kotlin("plugin.serialization") version "1.9.21"
+    kotlin("plugin.serialization") version "2.0.20"
 }
 
 repositories {