Gradle: Bump com.diffplug.spotless from 6.7.2 to 6.9.0

Bumps com.diffplug.spotless from 6.7.2 to 6.9.0.

---
updated-dependencies:
- dependency-name: com.diffplug.spotless
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

build.gradle.kts

@@ -26,7 +26,7 @@ plugins {
   kotlin("jvm") version kotlinVersion
   kotlin("plugin.spring") version kotlinVersion apply false
   id("pl.allegro.tech.build.axion-release") version "1.13.6"
-  id("com.diffplug.spotless") version "6.7.2"
+  id("com.diffplug.spotless") version "6.9.0"
   id("org.springframework.boot") version "2.7.0" apply false
   id("org.openapi.generator") version "5.4.0" apply false
   id("com.google.cloud.tools.jib") version "3.2.1" apply false