Bump com.diffplug.spotless:spotless-plugin-gradle from 6.20.0 to 6.21.0 (#3268)

Author: dependabot[bot]

buildSrc/build.gradle.kts

@@ -27,7 +27,7 @@ dependencies {
   implementation(gradleApi())
 
   // When updating, update above in plugins too
-  implementation("com.diffplug.spotless:spotless-plugin-gradle:6.20.0")
+  implementation("com.diffplug.spotless:spotless-plugin-gradle:6.21.0")
   implementation("com.github.spotbugs.snom:spotbugs-gradle-plugin:5.1.3")
   implementation("com.github.johnrengelman:shadow:8.1.1")
   implementation("com.gradle.enterprise:com.gradle.enterprise.gradle.plugin:3.14.1")