Bump com.diffplug.spotless from 6.20.0 to 6.21.0 (#539)

Bumps com.diffplug.spotless from 6.20.0 to 6.21.0.

---
updated-dependencies:
- dependency-name: com.diffplug.spotless
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

settings.gradle.kts

@@ -15,7 +15,7 @@
 
 pluginManagement {
   plugins {
-    id("com.diffplug.spotless") version "6.20.0"
+    id("com.diffplug.spotless") version "6.21.0"
     id("com.github.ben-manes.versions") version "0.47.0"
     id("com.github.jk1.dependency-license-report") version "2.5"
     id("com.github.johnrengelman.shadow") version "8.1.1"