build(deps): bump plugin.spring from 2.1.20 to 2.1.21 in /backend

Bumps [plugin.spring](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: plugin.spring
  dependency-version: 2.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

backend/build.gradle.kts

@@ -1,6 +1,6 @@
 plugins {
     kotlin("jvm") version "2.1.20"
-    kotlin("plugin.spring") version "2.1.20"
+    kotlin("plugin.spring") version "2.1.21"
 
     id("org.springframework.boot") version "3.4.4"
     id("io.spring.dependency-management") version "1.1.7"