Bump plugin.spring from 1.9.20 to 1.9.21

Bumps [plugin.spring](https://github.com/JetBrains/kotlin) from 1.9.20 to 1.9.21.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.9.20...v1.9.21)

---
updated-dependencies:
- dependency-name: plugin.spring
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

build.gradle.kts

@@ -6,7 +6,7 @@ plugins {
   id("org.springframework.boot") version "3.1.5"
   id("io.spring.dependency-management") version "1.1.4"
   kotlin("jvm") version "1.9.20"
-  kotlin("plugin.spring") version "1.9.20"
+  kotlin("plugin.spring") version "1.9.21"
 }
 
 group = "com.rogervinas"