dependabot: 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

@@ -61,7 +61,7 @@ plugins {
     id 'idea'
     id 'jacoco'
     id 'maven-publish'
-    id 'com.diffplug.spotless' version '6.20.0'
+    id 'com.diffplug.spotless' version '6.21.0'
     id 'checkstyle'
     id 'com.netflix.nebula.ospackage' version "11.4.0"
     id "org.gradle.test-retry" version "1.5.4"