Update plugin spotless to v6.21.0

Author: renovate[bot]

gradle/libs.versions.toml

@@ -25,4 +25,4 @@ r8 = "com.android.tools:r8:8.1.56"
 dokka = "org.jetbrains.dokka:1.8.20"
 kotlin-jvm = "org.jetbrains.kotlin.jvm:1.9.10"
 mavenPublish = "com.vanniktech.maven.publish:0.25.3"
-spotless = "com.diffplug.spotless:6.20.0"
+spotless = "com.diffplug.spotless:6.21.0"