build(deps): bump com.diffplug.spotless:spotless-plugin-gradle (#960)

Bumps [com.diffplug.spotless:spotless-plugin-gradle](https://github.com/diffplug/spotless) from 6.20.0 to 6.21.0.
- [Changelog](https://github.com/diffplug/spotless/blob/main/CHANGES.md)
- [Commits](diffplug/spotless@gradle/6.20.0...gradle/6.21.0)

---
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]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

buildSrc/build.gradle.kts

@@ -9,7 +9,7 @@ repositories {
 }
 
 dependencies {
-    implementation("com.diffplug.spotless:spotless-plugin-gradle:6.20.0")
+    implementation("com.diffplug.spotless:spotless-plugin-gradle:6.21.0")
     implementation("net.ltgt.gradle:gradle-errorprone-plugin:3.1.0")
     implementation("com.gradle.publish:plugin-publish-plugin:1.2.1")
     implementation("org.sonarsource.scanner.gradle:sonarqube-gradle-plugin:4.3.0.3225")