Docx4j 8.1.6 released

[jason]

docx4j 8.1.6 is now in Maven Central.

Its just a minor release:

- improve efficiency of package clone
- pptx4j: SaveSlides provides the ability to save selected slides only
- xlsx4j: handle revision namespaces written by recent Excel releases; support importing flat OPC XML representing a spreadsheet

To use it via Maven, just add one and only one of docx4j-JAXB-Internal, docx4j-JAXB-MOXy,
or docx4j-JAXB-ReferenceImpl to your pom. https://search.maven.org/search?q=g:org.docx4j

Otherwise, see https://www.docx4java.org/downloads.html

Minimum supported Java version is 8.0