Groovydoc fails to produce documentation for some files with higher Java language level code

Groovydoc supports both groovy & java files.  It supports parsing via the JavaParser library & the library supports specifying a language level.  However, groovydoc does not have a way of setting it for the library at this time.  By default the library defaults to the "popular" language level of that release - for the current JavaParser version that is Java 11.  

As projects update to newer language features, it is desirable to continue using groovydoc for later Java versions.  This change is to add an option to groovydoc to specify the language level based on the possible values in the enum ParserConfiguration.LanguageLevel from the JavaParser library.