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

Bumps [org.jetbrains.kotlin.plugin.allopen](https://github.com/JetBrains/kotlin) from 2.0.21 to 2.1.0.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v2.1.0/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.0.21...v2.1.0)

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

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

backend/build.gradle

@@ -25,7 +25,7 @@ plugins {
     id "pl.allegro.tech.build.axion-release" version "1.18.16"
     id "com.diffplug.spotless" version "6.25.0"
     id "org.jetbrains.kotlin.jvm" version "2.1.0"
-    id "org.jetbrains.kotlin.plugin.allopen" version "2.0.21"
+    id "org.jetbrains.kotlin.plugin.allopen" version "2.1.0"
 }
 
 ext {