Bump spotless-plugin-gradle from 6.8.0 to 6.9.0

Bumps [spotless-plugin-gradle](https://github.com/diffplug/spotless) from 6.8.0 to 6.9.0.
- [Release notes](https://github.com/diffplug/spotless/releases)
- [Changelog](https://github.com/diffplug/spotless/blob/main/CHANGES.md)
- [Commits](diffplug/spotless@gradle/6.8.0...gradle/6.9.0)

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

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

Author: dependabot[bot]

build.gradle

@@ -4,7 +4,7 @@ buildscript {
   }
   dependencies {
     classpath 'com.cinnober.gradle:semver-git:2.5.0'
-    classpath 'com.diffplug.spotless:spotless-plugin-gradle:6.8.0'
+    classpath 'com.diffplug.spotless:spotless-plugin-gradle:6.9.0'
     classpath 'io.github.cosmicsilence:gradle-scalafix:0.1.13'
   }
 }