Bump plugin.spring from 1.8.20 to 1.8.21

Bumps [plugin.spring](https://github.com/JetBrains/kotlin) from 1.8.20 to 1.8.21.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v1.8.21/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.8.20...v1.8.21)

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

Signed-off-by: dependabot[bot] <support@github.com>

build.gradle.kts

@@ -31,7 +31,7 @@ plugins {
 
     // Kotlin
     kotlin("jvm") version "1.8.20"
-    kotlin("plugin.spring") version "1.8.20"
+    kotlin("plugin.spring") version "1.8.21"
     kotlin("plugin.jpa") version "1.8.10"
     kotlin("kapt") version "1.8.10"