upgraded checkstyle plugin (security alert from github)

Author: wumpz

pom.xml

@@ -243,7 +243,7 @@
                     <dependency>
                         <groupId>com.puppycrawl.tools</groupId>
                         <artifactId>checkstyle</artifactId>
-                        <version>6.19</version>
+                        <version>8.12</version>
                     </dependency>
                 </dependencies>
             </plugin>