Bump kotlinVersion from 2.0.20 to 2.0.21

Bumps `kotlinVersion` from 2.0.20 to 2.0.21.

Updates `jvm` 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)

Updates `plugin.spring` 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: jvm
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: plugin.spring
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

build.gradle.kts

@@ -9,7 +9,7 @@ version = "3.3.0"
 java.sourceCompatibility = JavaVersion.VERSION_17
 
 plugins {
-    val kotlinVersion = "2.0.20"
+    val kotlinVersion = "2.0.21"
     id("org.springframework.boot") version "3.3.4"
     id("io.spring.dependency-management") version "1.1.6"
     id("com.google.cloud.tools.jib") version "3.4.3"