Update maven-pmd-plugin configuration to target JDK-11 (#63)

diff --git a/pom.xml b/pom.xml
index 00c017d..0aea717 100644
--- a/pom.xml
+++ b/pom.xml
@@ -404,7 +404,7 @@
                             <ruleset>${resources.base.path}cxf-pmd-custom.xml</ruleset>
                         </rulesets>
                         <sourceEncoding>UTF-8</sourceEncoding>
-                        <targetJdk>1.8</targetJdk>
+                        <targetJdk>11</targetJdk>
                         <linkXRef>false</linkXRef>
                         <includeTests>true</includeTests>
                         <verbose>true</verbose>