chore(deps): bump org.jetbrains.kotlin.jvm in /backend

Bumps [org.jetbrains.kotlin.jvm](https://github.com/JetBrains/kotlin) from 2.1.20 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@v2.1.20...v2.1.21)

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin.jvm
  dependency-version: 2.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

backend/build.gradle

@@ -24,7 +24,7 @@
 plugins {
     id "pl.allegro.tech.build.axion-release" version "1.18.18"
     id "com.diffplug.spotless" version "7.0.3"
-    id "org.jetbrains.kotlin.jvm" version "2.1.20"
+    id "org.jetbrains.kotlin.jvm" version "2.1.21"
     id "org.jetbrains.kotlin.plugin.allopen" version "2.1.21"
 }