[Backport 2.x] dependabot: bump com.diffplug.spotless from 6.20.0 to …

…6.21.0 (#3298)

Backport 7c61fd9 from #3295.

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build.gradle

@@ -61,7 +61,7 @@ plugins {
     id 'idea'
     id 'jacoco'
     id 'maven-publish'
-    id 'com.diffplug.spotless' version '6.20.0'
+    id 'com.diffplug.spotless' version '6.21.0'
     id 'checkstyle'
     id 'com.netflix.nebula.ospackage' version "11.3.0"
     id "org.gradle.test-retry" version "1.5.4"