Issue #542: update checkstyle to 10.21.1

Author: muhlba91

README.md

@@ -17,6 +17,7 @@ Compatibility matrix from Checkstyle:
 
 | Checkstyle Plugin | Sonar min | Sonar max | Checkstyle | Jdk |
 |-------------------|-----------|-----------|------------|-----|
+| 10.21.1           | 9.9       | 10.7+     | 10.21.1    | 11  |
 | 10.20.2           | 9.9       | 10.7+     | 10.20.2    | 11  |
 | 10.20.1           | 9.9       | 10.7+     | 10.20.1    | 11  |
 | 10.19.0           | 9.9       | 10.7+     | 10.19.0    | 11  |

pom.xml

@@ -93,13 +93,13 @@
   </ciManagement>
 
   <properties>
-    <checkstyle.version>10.20.2</checkstyle.version>
+    <checkstyle.version>10.21.1</checkstyle.version>
     <sonar.version>9.9.4.87374</sonar.version>
     <sonar.api-version>9.9.0.229</sonar.api-version>
     <maven.sevntu.checkstyle.plugin.version>1.44.1</maven.sevntu.checkstyle.plugin.version>
     <!-- it should be a version of checkstyle that is compatible/compiled with sevntu -->
     <maven.sevntu.checkstyle.plugin.checkstyle.version>
-      10.20.2
+      10.21.1
     </maven.sevntu.checkstyle.plugin.checkstyle.version>
     <antrun.plugin.version>3.1.0</antrun.plugin.version>
     <checkstyle.configLocation>https://raw.githubusercontent.com/checkstyle/checkstyle/checkstyle-${checkstyle.version}/config/checkstyle-checks.xml</checkstyle.configLocation>