pmd: 6.7.0 -> 6.8.0 (#47736)

* pmd: 6.7.0 -> 6.8.0

Semi-automatic update generated by
https://github.com/ryantm/nixpkgs-update tools. This update was made
based on information from
https://repology.org/metapackage/pmd/versions

* pmd: refresh meta

Homepage is now https://pmd.github.io/
Added licenses