Bump plugin.spring from 1.9.20 to 1.9.21 in /springboot-app (#71)

* Bump plugin.spring from 1.9.20 to 1.9.21 in /springboot-app

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] <support@github.com>

* Update build.gradle.kts

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Roger Viñas Alcon <roger.vinas@fastzink.com>

springboot-app/build.gradle.kts

@@ -1,8 +1,8 @@
 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("jvm") version "1.9.21"
+  kotlin("plugin.spring") version "1.9.21"
 }
 
 group = "org.rogervinas"