Bump org.springframework.boot in /chat-getting-started-kotlin

Bumps [org.springframework.boot](https://github.com/spring-projects/spring-boot) from 2.7.5 to 3.1.2.
- [Release notes](https://github.com/spring-projects/spring-boot/releases)
- [Commits](spring-projects/spring-boot@v2.7.5...v3.1.2)

---
updated-dependencies:
- dependency-name: org.springframework.boot
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

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

@@ -4,7 +4,7 @@ val axonVersion: String by extra { "4.3.3" }
 val swaggerVersion: String by extra { "2.9.2" }
 
 plugins {
-    id("org.springframework.boot") version "2.7.5"
+    id("org.springframework.boot") version "3.1.2"
     id("io.spring.dependency-management") version "1.1.3"
     kotlin("jvm") version "1.9.0"
     kotlin("plugin.spring") version "1.9.0"