Bump com.puppycrawl.tools:checkstyle from 10.20.2 to 10.21.0

Bumps [com.puppycrawl.tools:checkstyle](https://github.com/checkstyle/checkstyle) from 10.20.2 to 10.21.0.
- [Release notes](https://github.com/checkstyle/checkstyle/releases)
- [Commits](checkstyle/checkstyle@checkstyle-10.20.2...checkstyle-10.21.0)

---
updated-dependencies:
- dependency-name: com.puppycrawl.tools:checkstyle
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

pom.xml

@@ -28,7 +28,7 @@
     <pmd-plugin.version>3.26.0</pmd-plugin.version>
     <pmd.version>7.8.0</pmd.version>
     <checkstyle-plugin.version>3.6.0</checkstyle-plugin.version>
-    <checkstyle.version>10.20.2</checkstyle.version>
+    <checkstyle.version>10.21.0</checkstyle.version>
 
     <!-- SonarCloud setup -->
     <sonar.projectKey>SvenWoltmann_java-collections-guide</sonar.projectKey>