Bump maven-pmd-plugin from 3.20.0 to 3.21.0

Author: dependabot[bot]

pom.xml

@@ -184,7 +184,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-pmd-plugin</artifactId>
-        <version>3.20.0</version>
+        <version>3.21.0</version>
         <configuration>
           <targetJdk>11</targetJdk>
           <rulesets>