Bump plugin.spring from 2.0.20 to 2.0.21 in /chat-getting-started-kotlin

Bumps [plugin.spring](https://github.com/JetBrains/kotlin) from 2.0.20 to 2.0.21.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v2.0.21/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.0.20...v2.0.21)

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

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

chat-getting-started-kotlin/build.gradle.kts

@@ -7,7 +7,7 @@ plugins {
     id("org.springframework.boot") version "2.7.5"
     id("io.spring.dependency-management") version "1.1.6"
     kotlin("jvm") version "2.0.20"
-    kotlin("plugin.spring") version "2.0.20"
+    kotlin("plugin.spring") version "2.0.21"
 }
 
 group = "io.axoniq.labs.chat"