Update dependency com.diffplug.spotless:spotless-plugin-gradle to v6.21.0 (#9332)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

conventions/build.gradle.kts

@@ -54,7 +54,7 @@ dependencies {
   implementation("org.apache.maven:maven-aether-provider:3.3.9")
 
   // 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.google.guava:guava:32.1.2-jre")
   implementation("gradle.plugin.com.google.protobuf:protobuf-gradle-plugin:0.8.18")
   implementation("com.github.johnrengelman:shadow:8.1.1")

examples/distro/build.gradle

@@ -12,7 +12,7 @@ buildscript {
     }
   }
   dependencies {
-    classpath "com.diffplug.spotless:spotless-plugin-gradle:6.20.0"
+    classpath "com.diffplug.spotless:spotless-plugin-gradle:6.21.0"
     classpath "gradle.plugin.com.github.johnrengelman:shadow:8.0.0"
     classpath "io.opentelemetry.instrumentation:gradle-plugins:1.30.0-alpha-SNAPSHOT"
   }