Fix javadoc when releasing with java11

Due to changes the release has to be done with java 11.
This now failed during javadoc generation, with e.g.
```
error: package sun.nio.ch is not visible
```

pom.xml

@@ -118,6 +118,8 @@
 					<configuration>
 						<!-- Required for java8, so that javadoc errors don't fail the build. -->
 						<additionalparam>-Xdoclint:none</additionalparam>
+						<!-- Required for build with java11, to prevent "error: package sun.nio.ch is not visible" failing javadoc -->
+						<source>8</source>
 					</configuration>
 				</plugin>