Enable checkstyle for Java 11 (#154)

Java 11 has been required since the plugin first required Jenkins 2.361
or later.  If we're going to use checkstyle, let's use the most recent
checkstyle release.

pom.xml

@@ -116,16 +116,13 @@
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-checkstyle-plugin</artifactId>
         <version>3.6.0</version>
-	<!-- enable this when the plugin requires Java 11 -->
-	<!--
         <dependencies>
           <dependency>
             <groupId>com.puppycrawl.tools</groupId>
             <artifactId>checkstyle</artifactId>
-            <version>10.3.3</version>
+            <version>10.21.1</version>
           </dependency>
         </dependencies>
-	-->
         <executions>
           <execution>
             <id>checkstyle</id>