Bump com.diffplug.spotless from 6.20.0 to 6.21.0

Bumps com.diffplug.spotless from 6.20.0 to 6.21.0.

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

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

build.gradle.kts

@@ -13,7 +13,7 @@ plugins {
     id("org.sonarqube") version "4.3.0.3225"
     kotlin("jvm") version "1.8.21"
     kotlin("plugin.serialization") version "1.8.21"
-    id("com.diffplug.spotless") version "6.20.0"
+    id("com.diffplug.spotless") version "6.21.0"
     `java-library`
     `maven-publish`
     signing