Bump spotless-maven-plugin from 2.20.0 to 2.21.0

Bumps [spotless-maven-plugin](https://github.com/diffplug/spotless) from 2.20.0 to 2.21.0.
- [Release notes](https://github.com/diffplug/spotless/releases)
- [Changelog](https://github.com/diffplug/spotless/blob/main/CHANGES.md)
- [Commits](diffplug/spotless@lib/2.20.0...lib/2.21.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

pom.xml

@@ -88,7 +88,7 @@
         <maven-surefire-plugin.version>3.0.0-M5</maven-surefire-plugin.version>
         <maven-compiler-plugin.version>3.9.0</maven-compiler-plugin.version>
         <versions-maven-plugin.version>2.8.1</versions-maven-plugin.version>
-        <spotless-maven-plugin.version>2.20.0</spotless-maven-plugin.version>
+        <spotless-maven-plugin.version>2.21.0</spotless-maven-plugin.version>
         <gitflow-incremental-builder.version>4.0.0</gitflow-incremental-builder.version>
 
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