chore(deps): update plugin com.diffplug.spotless to v6.21.0 (#932) (#933

)

(cherry picked from commit 8a7b967)

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build.gradle

@@ -36,7 +36,7 @@ buildscript {
 
 plugins {
     id 'java-library'
-    id "com.diffplug.spotless" version "6.20.0" apply false
+    id "com.diffplug.spotless" version "6.21.0" apply false
     id 'jacoco'
     id "com.form.diff-coverage" version "0.9.5"
     // for javadocs and checks spotless doesn't do