Bump com.diffplug.spotless:spotless-plugin-gradle from 6.20.0 to 6.21.0

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]>

buildSrc/build.gradle.kts

@@ -43,7 +43,7 @@ tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile>().configureEach
 
 dependencies {
     implementation("com.github.spotbugs.snom:spotbugs-gradle-plugin:5.1.3")                // https://plugins.gradle.org/plugin/com.github.spotbugs
-    implementation("com.diffplug.spotless:spotless-plugin-gradle:6.20.0")                   // https://plugins.gradle.org/plugin/com.diffplug.spotless
+    implementation("com.diffplug.spotless:spotless-plugin-gradle:6.21.0")                   // https://plugins.gradle.org/plugin/com.diffplug.spotless
     implementation("gradle.plugin.org.kt3k.gradle.plugin:coveralls-gradle-plugin:2.12.2")   // https://plugins.gradle.org/plugin/com.github.kt3k.coveralls
     implementation("org.javamodularity:moduleplugin:1.8.12")                                // https://plugins.gradle.org/plugin/org.javamodularity.moduleplugin
     implementation("io.github.gradle-nexus:publish-plugin:1.3.0")                           // https://plugins.gradle.org/plugin/io.github.gradle-nexus.publish-plugin