Explicitly define source=8 for javadoc plugin

[bmarwell]

From #24 to fix this warning

[WARNING] javadoc: warning - The code being documented uses modules but the packages defined in https://docs.oracle.com/javase/8/docs/api/ are in the unnamed module.

Happens when compiling with Java 9+.
Either don't use source and target (impossible due to parent) or define source again for the javadoc plugin, or define release (only when compiling with 11+).