⬆️ Bump plugin.spring from 2.0.21 to 2.1.0 in /backend

Bumps [plugin.spring](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: plugin.spring
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

backend/build.gradle.kts

@@ -6,7 +6,7 @@ plugins {
     id("org.springframework.boot") version "3.4.0"
     id("io.spring.dependency-management") version "1.1.6"
     kotlin("jvm") version "2.0.21"
-    kotlin("plugin.spring") version "2.0.21"
+    kotlin("plugin.spring") version "2.1.0"
     kotlin("plugin.jpa") version "2.0.21"
     id("org.jlleitschuh.gradle.ktlint") version "12.1.2"
 }