Update spotbugs-maven-plugin to version 3.1.9 and enable SpotBugs on …

…Java 12+ (closes #99).

pom.xml

@@ -52,7 +52,7 @@
     <checkstyle.plugin.version>3.0.0</checkstyle.plugin.version>
     <checkstyle.version>8.14</checkstyle.version>
 
-    <spotbugs.plugin.version>3.1.8</spotbugs.plugin.version>
+    <spotbugs.plugin.version>3.1.9</spotbugs.plugin.version>
 
     <commons.jacoco.version>0.8.2</commons.jacoco.version>
 
@@ -461,8 +461,6 @@
       <properties>
         <!-- currently fails with "javadoc: error - The code being documented uses modules but the packages defined in https://docs.oracle.com/javase/8/docs/api/ are in the unnamed module." -->
         <maven.javadoc.skip>true</maven.javadoc.skip>
-        <!-- spotbugs does not work on java 12 yet, "java.lang.IllegalArgumentException: Unsupported class file major version 56" -->
-        <spotbugs.skip>true</spotbugs.skip>
       </properties>
     </profile>
   </profiles>